Hi - exTA (male) here :
I was a TA for 12 years, but started as a parent helper when my DS started primary school. I worked mostly in KS1, but also with all ages from nursery to Yr 10 in a tough comprehensive.
Assuming the teacher is a good one, she should give you all the guidance you need. Just support the children AND the teacher in whatever lesson/activity is taking place. In particular, look out for things the children may get up to behind her back; some teachers seem to have eyes in the back of their head and bionic ears, but if you do see poor behaviour or slacking don't be afraid to quietly intervene.
If there are any SN kids, or less able ones, they might be most deserving of your help. When supporting reading, don't tell a child a word they don't know, but try to encourage them to sound it out (using whatever phonics/decoding system the class uses). Similarly in other subjects, try to point them towards the answer so they can work it out for themselves. Sometimes a child might give an incorrect answer because he has misheard or misunderstood, but the teacher may not pick up on that. Also, gently encourage the shy, reluctant children to give answers. Give plenty of praise for good work, but don't stop to correct every incorrect detail, particularly if a child is really trying their best.
Any special skills you have that can be used in a school context might be appreciated by the teacher - art, music, science, sport, etc - but don't offer this straight away or you might be seen as too 'pushy'!
The voluntary helper can be in a fortunate position to have time to observe children, noticing any who seem unhappy, worried etc and giving them a sympathetic 'ear'. Probably the best thing I ever did was in a term as voluntary helper, between two employments. A Yr 1 Down's boy was fascinated by computers, but his lack of fine motor skills prevented him using mouse or keyboard. I found a learning website for people with disability, that only required him to press the space-bar, and this he could manage. With the permission of his teacher I let him use the various activities: making a rocket take off at the end of the countdown; jumping across a chasm by pressing the space-bar at the correct moment; creating patterns and matching shapes, etc. He was so thrilled to be able to control the computer for the first time, instead of just having to observe other children doing it; in a small way it changed his life, and it also boosted his skills so that his hand control quickly improved, and he could use the computer in the normal way.
Good luck, and enjoy!