Where to begin is pretty difficult to answer but if you do a search here on the forum you will find a LOT of valuable information. I have a basic startup checklist you can look at here:
http://www.billerswebsite.com/startupchecklist.htm KNOWLEDGE can sell JUST as good as experience IMHO. Yes it can be more difficult and you will need to CONTINUE education, and stay on top of the trending changes taking place. I think if just starting out, even though you are familiar with mental health, it's best NOT to target market right away. You will reduce your opportunities significantly. Later on after you have signed one or two mental health practices and you still want to specialize it will be a choice to consider. It can take a while to land that first client so you want to be sure to maximize and diversify your marketing efforts! Today's medical billing company can reduce their up front/start up costs also by marketing and utilizing the clients "existing" PM solution. If a PM solution is not in place or the client wants to make a change, you can give them various resources and let them choose and sign up/purchase the solution and you can access it remotely. Years ago medical billing companies were going out and spending big bucks on software and then had to not only sell their services but their systems as well. 9 out of 10 practices you talk with, will already have an existing solution in place.. why not cut your costs at same time. Remember your offering up your services. Use that money instead for a good marketing campaign.
Also, use the company you did your externship with as a reference. That indeed should serve as a reference and experience.
