It is useful to know what the main points of focus are in order to gain the maximum benefit from your practice.
When learning t'ai chi it is good to remember that the first lessons may be much more demanding than later ones. It takes time to learn to relax both about being in a class and whilst practising.
Don't use force, either mentally or physically. Allow yourself to be true to your experience, be authentic. This will make it easier to manifest the ch'i and it will also take away any strain that comes from thinking that things should be a certain way. Just relax as is.
Try to concentrate but also recognise that the mind gets tired just like a muscle. When your concentration feels forced take time to rest. How do you rest your mind? Think of nothing.
Keep the lower back, or sacrum, vertical. In order to do this you will need to leave the legs relaxed. In a stance where one foot is forward, feet shoulder wide, the back leg must be bent with the knee dropped. This will take the pressure off the lumbar spine and prevent compression in that area which might lead to back ache.