Firstly I wish I had started at 18/19 as during that time I had time and internet access without having to pay rent, so could have saved up a fair amount or just enjoyed spending cash on trivial things.
Secondly I wish I had done more research before starting, I wish I had watched more models and had taken more time about starting "right".
Thirdly I wish someone had said "don't expect or even aim to be a top model as you'll generally be disappointed. Just be happy with your own successes, work hard and avoid comparing yourself to others." Top models earn an insane amount of money, but just the top 1000 models on MFC earn the kind of money most people consider good earnings.
And erm, fourthly, I think a more straightforward attitude about not giving attention to controlling arsehole members. As a newer model you tend to attract the kinds of guys who go around "helping" models, which generally means being demanding, expecting extra attention and giving you shit advice which they think is excellent, and then generally being put down that you aren't achieving what they believe are levels of success. These types of members actually made my first experiences of camming a lot more unpleasant and I think lent me a pretty negative attitude towards it.
I would also say personally for me, that I wish I had got a part time job in an office environment. Partially for the company as camming can get very lonely, but also so you know you have some non camming related income and to put in perspective the hours needed in a regular job, and also to keep the CV looking good for if/when you decide to move on.