I agree with ticklemern. Start with yourself. It is really not very complicated, but when you don't know what you are doing, it seems so! What you are selling is yourself. How you do that is with your profile. Every agency completes a profile they send to hospitals to market travelers for open positions. It consists of a work history, skills checklist, two written references, and ideally a one page concise summary of the traveler - this includes the proposed start dates and contact info for an interview. All in all, only five well done pages - no manager wants to dig through a 20 page profile.
The best is to start with a travel contract for 13 weeks. You don't have the resources to be a registry, trust me. I don't, and I've had my own agency for 8 years. I would talk directly to a manager you've worked for, or one you can get on the phone and see if they want someone with your skill set (that usually turns into a mini interview. If so, now you can send your profile to HR (as an agency) and use the manager's desire to hire you as leverage with HR. If they agree to do business with you, you can send them a contract after negotiating a bill rate. Usually HR will tell you what they want to pay. It should be a fine rate for your first time, but feel free to negotiate if you want.
Your contract doesn't have to be perfect, and many times the hospital will ask for changes. Once that is signed, you will have to provide a number of other documents. You need professional and general liability, 1/3 is standard. They may ask for proof of workers comp and unemployment (not so many hospitals ask for that, but I'm always ready if they do). That's it for the agency side (no "license" in most states)! For the traveler side, you will need a physical, titers and immunizations, background check, drug screen, and licenses and certifications. Then go to work.
You will invoice weekly from your signed timecard (you need a form for that too). That takes like 5 minutes a week. The hospital, per your contract, will take 30 days to pay.
Obviously, there are lots of fine details. But that is the gist of it, it is really that simple. Just make a mental picture of you, the agency, negotiating for you, the traveler. Get your head around that and you will be fine. A large number of agencies start with a nurse or a tech representing themselves and grow from there. If you want to grow. I occasionally hire a traveler, but for the most part I make much more money just working assignments on my own. Margins on hiring other workers are not so great, and you need a lot of nurses to make it worthwhile just hanging out in an office.