I've been a model on and off since I was about 14, done over 50 shoots, worked semi professionally for about 3 years and really enjoy it.
Since I mentor a few younger girls in various things, I sometimes get emails from girls who want to start modelling and so here are a few tips I've picked up along the way. Also I hear a lot of photographers who really dislike specific things about new models.
Decide early what you will and won't do. A lot of new photographers, the kind you will be working with when you first start out will want to do nude or implied nude, as they think it automatically makes them seem more artistic. I will do implied nude, but only on a professional job. If a photographer or company isn't willing to pay for nudity then don't do it.
When setting up a purpleport or model mayhem account don't lie. I'm 5ft 5, so not normal agency standard, and I still work so there is no need to lie, and photographers will know, it's their job. If you're lucky enough to have agency standard measurements, I highly suggest you go for an agency. I've had a few over the years although I'm currently independent until I move to London. I can recommend premier, elite and storm, but if you're under 5ft 7, don't waste their time, contact petite specific agencies or promo agencies.
When doing TFP be clear about what you want. A good photographer worth using will provide 2 or 3 fully edited images. If you have done a shoot and the photographer has given you 300 images, chances are they weren't very good, in this case, ask the advice of another photographer which images you should use.
Network. This is so important, in my area there are networking events for models, photographers and MUAs. I personally have a make up artist who I like to use, and it just makes things safer when going for shoots, and also means we can share contacts.
Take care of yourself. Your face and body are now your job, so develop a skincare routine, workout, and just generally be happy with yourself. I'm not a skinny minnie model, and I have huge boobs, so I get a lot of pro underwear work, however I know if I eat rubbish every single day, it will affect my skin, eyes, hair and body. Eating unhealthily is also terrible for my depression. So just keep an eye on it, and no starvation diets girlies!