How can i do this

Well, there's a guy I work with that sews his own bears for a summer time job, and he makes 3-4 of them a day. I haven't really talked about it too much with him because I don't really work with him too much, but I believe he uses patterns for the bear, and you can scale them for whatever size you like, then it's just a matter of being able to sew them together. If you like I can talk with him more later about it.

Google how to make a stuffed bear...or bear sewing patterns.

Country Handcrafts puts out Teddy Bear books and patterns that are really cute. Look at the competition, like Vermont Teddy Bears, and see what themes they offer. I am sure there must be online groups for Bear lovers. One guy I worked with had an entire room of Beenie Bears. (another story)
Visit a Build a Bear workshop to see their process. It should give you ideas.
Make a variety of sizes and use joints in some as well. Your local library is a good source, crafts section, or toymaking books, as well as doing an online search.

I wanted to make teddy bears too! I found a very simple pattern. It is only 4 pieces and is easy enough for a beginner to sew. The pattern has a bear, bunny, and a dog, so you can make a bunch of different ones for friends and family too! If your town doesn't have a fabric store, you can purchase a cheap micro-fiber blanket for a very soft animal. Here is a link to order the pattern online if you can't find it in stores: Simplicity Crafts Pattern NO: 9524

http://www.simplicity.com/Search.aspx?SearchTerm=9524&x=0&y=0

There is an easy 2 piece pattern from either mccalls or simplicity i think. Check out the fabric stores when patterns are on sale :)