Hi Nic. What breed of sheep do you have? Polled Dorsets (which I keep) can lamb at any time of year as do a small number of other breeds. However, the usual lambing time is spring (ish) this can be as early as late December and as late as May. This is because most sheep have a breeding 'season' which lasts from August(ish) to late November with a gestation period of around 158 days (give or take 10-20 days depending on breed). So moving to your questions:
1. If the sheep are looking thin, it could be because they've just lambed. They will definitely need some ewe nuts as they need to produce milk to feed their lambs as well as themselves. Might be an idea to get them a decent mineral lick as well - all available from your farm supplier who will advise you on the best product.
2. The lambs can be weaned any time after 12 weeks of age, although I usually leave it longer and sometimes, if there is no reason to wean them, I will leave them with their mothers for as long as possible. Some will do this differently. But that said, ensure they are weaned well before you put the ram in with them next time, if you bother to breed this year, so they can be in top condition in time to lamb again.
3. The lambs can be given creep after around 4 weeks of age. If you are fattening them to eat then this is definitely advisable.
Hope this helps and the best of luck.