I have a few collections that I've started over the years.
My favorite would probably be my postcard collection. I don't remember when I started collecting postcards, but I'm guessing I was around seven. Since then I've had to get postcards on pretty much every trip I've been on. Whether that was a day trip or something much longer. I have NO idea how many postcards I have now, but it's a lot. I also love buying vintage postcards from thrift shops. I have a really awesome black and white one from France right after WWII. It's even got a message on it to someone. Very cool!
I collect Barbie's too, even though I haven't really added to my collection much the past few years. I, of course, have all my Barbies from when I was little, but I don't really count those towards my collection. I have 2 Rubbermaid containers full of all my Barbies and that's not even counting all the Barbie ornaments I have. I have so many Barbie ornaments that I am going to do a tree with just them one of these days.
What do you collect?

I also collect vintage Nancy Drew books. I have a decent collection of them, most of them I received as gifts for birthdays and Christmas. Most of them are from the 1930s. I'm still on the lookout for them when I go to old book sales or antique shops in the States.