How can I sell handmade things online?

Etsy is going to be your best bet, I think. There's a $0.20 listing fee for each item you list, but this is one of the lowest fees you'll find online. Contrary to what a previous person said, though, there is a 3.50% transaction fee for any items you sell - which, again, is pretty small. So far, the best luck I've had with selling on Etsy are thin (about 1.5 to 2 inches in width) loom knitted scarves. I don't think I've actually sold any crocheted items yet... I've heard a lot of other people have had a lot of success selling crochet and knitting patterns, though, which you may want to think about. It's a lot cheaper, easier, more profitable, and less of a hassle (being able to e-mail your product as opposed to shipping). One thing to remember, though, is that, because you're still a minor, you're legally required through Etsy's terms of use to use Etsy only in the supervision of a parent or guardian (and this is going to apply to most, if not all, other websites as well).

You have to pay a fee, but some good places are etsy and ebay.

You can post them on free classifieds, art and craft section, http://www.houseofnothing.com