Good for you OP and well done. Getting someone to be chair of the PTA is often quite hard work.
I did it for two years, and am still involved a lot in the PTA. I think that it varies hugely from school to school. Our school is a four form entry so quite big.
I don't think there are many legal considerations you need to think about. If your PTA earns more than £5K a year (do check this) it does need to be registered as a charity. PTA UK will help with that. Always make sure you get a licence for any event with alcohol - they're easy to do/get and don't cost much.
You will need to have a consitution if you're registered as a charity, and you'll need to have proper accounts. Your treasurer will handle those though.
I'd say the important thing as chair is to make sure you're not the person organising everything. If someone has an idea then encourage them to be the lead on it, with support from you.
Have regular meetings with your head. We have a standing fortnightly slot with our head where we go to just run over things. I always encouraged parents involved in the PTA to come along too, it gives them an idea of what the school is like etc. It is important that these meetings are kept on a PTA footing though - and not for individual disagreements etc. Anything like that parents must take themselves to the head.
We just cover events we want to hold, ways we can support the school and also a parent perspective. For example, a new gate was put on the alley that ran down to the Reception playground where parents went to collect children. It had been hinged in a way that meant it no longer opened wide enough for push chairs to go through easily, and was creating a bottle neck. Letting the head know this meant it could get rehung - teaching staff wouldn't necessarily see something like that.
We also tried to make the PTA as inclusive as possible. But again you get the same people who step forward, and you do tend to chat to people you know, so it can look cliquey even when it isn't. It is a bit of work to make it as open as possible.
During the year we do quiz nights, bingo nights, zumba and belly dancing fitness classes, a cake/second hand book & uniform sale at the end of each half term, a summer fair, and we do a second hand uniform sale at the inductions for nursery and reception. We also run refreshments at sports day. And the biggie - our summer fair. We also put on the Year Six Leavers disco, an easter bonnet parade for the infants, and a model competition for the juniors.
Many of our members work, and we've found that evening meetings held at 8pm at a nice pub often go down well.
Also as much notice as possible helps. So having meetings noted in advance in the calendar on the newsletter helps.
Being on the PTA can be a lot of fun, and a great way to get to know the school and other parents. Teaching staff have been hugely grateful for the time that we spend - they know we are giving up our spare time to do this.
But at the end of the day there's no way to do a bad job as chair of the PTA. Because if you weren't doing the job it wouldn't be done. It's an unpaid, voluntary position and it is really lovely you're taking it on.