I got Abby when she was 10 weeks old, and I took her to the vet pretty much straight away to get her checked. The vet told me that they should get wormed every month until they're 6 months old, but I don't really know when you should start! she had a few fleas when I got her (the vet said it is pretty normal for a small cat, and she didn't have many anyway) so I gave her some tablets specifically for kittens, and now I use Advocate spot on treatment, recommended by my vet, and so far is working ok! She's just over 4 months now, and I do let her out nearly every day, always supervised, and luckily she didn't get fleas anymore!
When I took her to the vet, I used a heated pad (specific for pets, can't remember the name now) and wrapped her basket with a blanket, since it's a wicker basket so i didn't want her to get too much wind/cold. To be fair, now she keeps on going out even if it's cold, and doesn't seem to have any problems.. she'd stay out all day if it was for her!
good luck with your kitten!!!!