I suppose there is no real set thing I can say to start modeling. Basically my advice would be to do something simple, but at the same time, something that your really interested in. That way its easier to stick with it when it gets frustrating! For example, I am a big Lord of the Rings fan, and doing the Fell Beast has been tough(its my first proper organic model!), but because I really love the subject matter, I stick with it and try to make it look as good as possible, and I have learned a lot from modeling it.
Don’t get disheartened if things go wrong with it, it happens all the time! The way to learn 3D modeling is to get stuck in and make mistakes…. ;)
I would also recommend heading over to 3DBuzz and downloading some of their video tutorials, they are absolutely fantastic and will teach you a LOT. A lot of the videos are free to download to everyone, but some others are only available to download to people who are actively part of the community, ie: post and participate in their forums(which are also great for learning and picking up tips etc).
So basically:
1. Create things that interest you
2. Don’t be afraid to make mistakes
3. Download as many tutorials as you can, and read as many books\articles as you can.
If you have any other questions, feel free to ask! :)