I started as a parent helper when my DS was in Reception - went on to become a TA (male) in another school for ten years in KS1, then two years in Secondary. Now retired, I do two days a week reading support in a small Primary, only three classes and mixed Year Groups.
It is certainly beneficial for a school to have parent helpers, and you will improve as you gain experience, just as the children do. You do need, I think, advice either from the class teacher, or the Literacy Coordinator, as to exactly HOW to support the children. I could explain my strategies, but then your school might work in a different way. But it is essential, if at all possible, that a child ends the session feeling GOOD about his efforts/performance.
So, as you probably know, PRAISE and ENCOURAGEMENT are important. Personally, I like to spend 10 to 20 minutes with each child, and I question, explain, compare etc as much as possible. For example, last year I had a Reception boy who was fascinated by volcanoes; at the end of lunchtime I sometimes gave him five minutes looking at volcanoes on the internet. He is now Year1 and recently his class teacher told him to show me a couple of books he was looking at in addition to his 'scheme' reader; they were atlases, and he was enthralled looking at maps, and I explained the colour key for heights of hills. I think experiences like that are invaluable, and the topic could just a easily be nature, science, crafts, sport - whatever builds a 'bridge' between adult and child.
The voluntary helper is in a privileged position, having TIME to spend with a child, which no teacher, nor even TA possibly, can match. Many children get very little quality time with parents or other adults at home, and for some kids school is the secure supportive environment they need.
Regarding what to 'correct' for a child: I don't think it is necessary to correct every mistake, but some children do have a 'pattern' of errors they make, so that can be pointed out. Rather than tell them the correct word, try to help them work it out for themselves. If they say an incorrect word, ask them what sound that incorrect word starts with. Chances are they will realise something is wrong, even if they can't correct it straight away. Thus, the correct word might be COTTAGE, but child says HOUSE because he has looked at the picture. Ask if he knows how to spell house; even if he doesn't he will probably realise it bears no resemblance to cottage.
Reliance on pictures is very much discouraged these days, but I think can be used if a child has almost sounded a word, but can't quite get there. Today my Yr2 reader got stuck on DONKEY, he had the component parts, but couldn't quite put it all together; the picture clinched it for him!
So - enjoy your time with the children, and I'm pretty sure they will enjoy, and benefit from, the support, respect and entertainment you give them.
cheers