It use to be that you had to be 5' 8" or taller weight....the skinnier the better. I used to model and I'm a little over 5' 8", I was more on the curvy side so I weighed and 114 lbs. For other tips I would say, BE CONFIDENT!!!! put on 2 or 3 layers of skin. Most of all have fun.
There are many types of models. Runway models are always crazy skinny and tall, but for catalogues they don't have to be. It really depends on what clothes you're modeling. As long as you aren't very fat you should be fine.