Welcome to the world of Taijiquan (T'ai Chi Ch'uan as it is commonly referred to in the West.) You are probably a complete beginner so I would offer three recommendations to you:
All the early benefits of Taiji rely on this one key skill. Wherever you feel your body stiffen, try and relax. Train yourself to recognise when your body is stiffening - then learn not to stiffen. The progress you make will be directly proportional to your ability to do this. The service that your teacher should provide is to help you understand when your body stiffens, to show you where your body is stiff and help you work out how to loosen it.
Unless you are training to fight, Taiji should be fun - hard work also at times but always inspiring, thought provoking and a labour of love. If it becomes a burden examine your perspective or put your Taiji aside. Try and understand what it is you want out of Taiji and keep your eye on your progress.
As a beginner, if it hurts you are probably doing it incorrectly! As a beginner pain nearly always comes from a twisted joint. You are looking for balance. While the process of becoming stronger, physically, mentally, psychologically or spiritually is fraught with pains of sorts, invariably they are indications of stepping from the path not along it. Generally speaking ignoring pain is a short cut to long term problems and damage. If you have regular pain then your main focus should not be on learning more postures but on learning to stand and then to move without pain.
If you keep these three somewhere in the back of your mind you won't go far wrong.
In my experience most people who come to study Taiji are not interested in the martial arts. They are interested in relaxation, stress reduction, centering/grounding, healing, improving physical conditions, helping concentration, improving mental, physical and spiritual balance, power and an interest in things Eastern, Chinese, spiritual or esoteric. All these things are fine aspirations for the study of Taiji. However most people consistently make a serious, erroneous assumption: that these interests can be achieved within Taiji without building internal strength.
For most people strength equates with hardness: they equate hardness with stiffness. In fact many equate hardness with the opposite of the attributes which they seek. In Taiji this is not true. In all the internal arts a practitioner is hard or soft as the situation warrants. However, a Taiji practitioner should not become stiff. A certain quality of looseness or relaxation is central to the principles of Taiji. This looseness (the Chinese word is sung) is not limp or dead but alive and vital. Perhaps the analogy of a sleeping tiger or a calm ocean might give a better image: or a tree, which bends with a strong wind without loosing its root. This strength is both of mind and body. This looseness must be capable of being simultaneously strong, otherwise it has no use. Hardness and softness are merely a demonstration or concealing of this strength.
Standing practice (sometimes referred to in Taiji as meditation) is sometimes practiced in a class to help achieve some basic mind and body skills in a simple physical state, i.e. standing still. Trying to relax the body while keeping it upright and to balance the relaxation of muscles over the whole body is difficult for most people to begin with. Many people have difficulty first of all trying to quieten their mind. Trying to do these while moving is understandably more difficult.
Initially it is recommended that the simplest of standing postures be assumed to reduce difficulties and to allow the correct understanding of mind and body. This is known as the Wuji position. The feet are apart, with ankles under shoulders, legs straight but not locked, head upright (gaze forward,) standing as though balancing a light weight on the crown / top of your head and letting that weight be transferred to the ground under your feet while keeping your body relaxed. Several different standing postures are used later. (All the Taiji postures can be used as standing postures to develop correct jin or strength/skill).
Your instructor may come round and correct your body position. Try and feel how your body position has been changed. They may put you in a position that feels as though you are leaning forwards. (This is common as most people habitually lock their knees, which tilts their pelvis forward and causes them to lean slightly backwards.) Be aware that any discomfort may be simply your mind saying you are in an unfamiliar position. Feel if your body is more relaxed after it has been adjusted. If anywhere still feels tense and you cannot relax there let your instructor know. At this stage standing should be about becoming aware of habitual postural errors and quietening the mind, not muscle conditioning. In summary, improving relaxed balance.
This "feeling what it is like" is often referred to as "Listening Inside." This is a particularly helpful skill to develop. It greatly increases your ability to obtain bodily feedback. This helps initially in learning where the body is tense. (The modern western term "Proprioception" describes the ability to understand what the body (muscles) are doing: to bring it into conscious awareness rather than unconscious habit.)
As you progress in your Taiji, if you like to practice standing meditation at home, and it is to be recommended to those who have the time, try holding one posture from the previous weeks study for that one week standing practice. A few minutes are fine. Try and feel a path from the part of your body being used through the middle of your body to the ground, all the while keeping your body relaxed and making minimal use of postural muscles. Use your mind to project up, out or down in the direction you want to be strong.
The priorities in standing meditation are:
1. Relaxing the body while keeping appropriate balance and structure (maintaining Peng Jin or Ground Strength).
2. Correct use of the mind - initially quietening the mind and then using the mind to focus, directing the "Peng Jin" or Ground Strength in the appropriate direction: Up, Out (away from the body), Down or In (towards the body).
The first steps in Taiji are:
1. Learning where the body is stiff
2. Learning how to practice relaxing (loosening exercises)
Without all three of these no major benefit will come from your practice.
The loosening exercises are there primarily to do that: loosen the body, relax the joints and bring some awareness to problem or potential problem areas of the body. (Initially some exercises may just be used to mechanically loosen stiff joints. This needs to be done before more "internal" type activity can be practiced.) They teach some elemental leg and torso conditioning and improve basic co-ordination and balance. Some of the exercises are based around postures to be found in the form. All work to improve awareness in specific joints using particular movements. Try to focus on that one thing in that exercise.
Generally the exercises start with the stiffest parts of the body, the back/waist, chest, hips and shoulders. Try and learn how and where to move them so that they remain loose at all times. Try not to loosen one joint at the expense of stiffening another. As a beginner, at first it may be difficult to see what is going on. Ask questions. If one exercise is particularly troublesome try asking your instructor to watch you or help you personally. Try then to practice it in your own time.
1. Turning the Waist horizontally - initially keep your hips relatively immobile and turn your waist from side to side. (This is trying to loosen the waist relative to the hips.) Do not mistake turning the hips for turning the waist.
2. Turning the Waist vertically forward / Bowing the Back - Keep your back loose as it flexes. Rotate your pelvis and midsection rather than straightening and bending your legs. This exercise is to aid loosening and subsequently strengthening the back. Eventually rotate the waist directly by relaxing and straightening your lower back
3. Turning the Waist Vertically side to side - imagine sliding your left palm down the left side of your left thigh to touch your left knee without bending your legs or leaning. Then try on the right side.
4. Loosening the Chest or Open and Closing your Chest: without shifting forwards or backwards, flex your upper spine so that you push out with the middle of your chest and then push back with the upper-middle of your back
5. Loosening the Hips and Shoulders - rotate your arms and legs to loosen your shoulders and hips.
Generally speaking try and keep the body as relaxed as you can. Identify where your body is stiff and work to loosen it. These exercises are designed to highlight and help loosen the stiffest parts of the body.