I have to said this first,
DO NOT EXPECT TO MAKE A LIVING IN THIS SPORT.
If you're serious about fighting, want to prove how good you are or will be; read on.
Like a struggling musicians, it is hard to make any money, don't let the financials get you down, your chances of making a living strictly by fighting is very very slim. So you have to be really serious about being a pro MMA fighter, expect to be great, not to get a lot of money.
1st, Look for a gym near you that trains specifically in mixed martial arts. Some of you may have to move to a new area, (or country if you not in US, UK) to area that has a lot of MMA gyms to chose from. If you move away from your family/friends/hometown, then you will have no other options but to stay focused and train. Also, you need to find and live in a tiny place where the rent is absolutely cheap. That way, you can train full time, and work a part time job in between training sessions, or during your days off from training.
Choose reputable MMA gyms depend on your financial status.
If you have a lot money, you can train at a very reputable Brazilian Jiu-Jitsu dojo, as well as a very reputable muay thai or kickboxing academy. Also, hiring a nutrionist would be a very good idea. Keyword is Money is the limit.
How to look for a gym near you that trains specifically in mixed martial arts ? They will usually advertise in the Yellow Pages with a list of the techniques they specialize in. Try to find one that has at least a couple of fighters on staff as their experience will be invaluable to you.
If you cannot find a gym that specializes in mixed martial arts, consider studying Muay Thai, also known as Thai Kickboxing, boxing or other striking arts. For grappling skills study Brazilian Jiu-Jitsu, Judo, sambo and wrestling. You can also purchase fighting technique DVDs or books to enhance your study.
2nd, Try to play off your natural strengths. Try to play off your natural strengths. If you have solid punching power, consider focusing on striking. If you have wrestling experience from high school or college, focus on your ground game. That being said you need to know both striking and grappling as most gyms train their fighters in a balanced style.
3rd, Know your weaknesses as well. If you don't have a solid chin, try to get the fight to the mat to eliminate the odds of getting knocked out. Likewise, if your opponent has poor striking skills but is skilled on the ground, you may want to keep the fight on your feet.
4th, Become familiar with how to defend yourself outside of your own discipline. Due to the nature of cross-training, all strikers must know some grappling and vice versa.
5th, Practice, practice, practice. Don't get discouraged. The more you "tap out" or get punched out, the more you learn. You have to love what you're doing, if you don't love it, quit and save yourself your monthly gym dues and your time. Train to the fullest of your potential and train on strength and conditioning.
6th, Enter as many competitions in kick boxing and grappling as you possibly can. Realize that every time you lose, you get better.
7th, Enter a few amateur Mixed-Martial-Arts competitions. Make sure that the promoters don't charge you a high "entry fee" as they are making money off you; make sure that they match you with someone of comparable skill.
M. M. A. promoters are sometimes present to recruit you if your skills are promising, for example, The Ultimate Fighter. However, those fighters have been fighting for years and are well established. It is neither as quick nor as easy as it once was.
If you're serious about fighting, remember that theres hundreds, if not thousands of potential opponents who are as serious, if not more serious than you. If you haven't noticed, MMA is getting huge, and the level of competition is enormous... So in order to hang with the big boys, you need to be at the same level as them.
There is no set requirement, but it is better to get a few in, at least 3 amateur fights are you suppose to have or required to have before you start having a professional record.
If you go into pro fighting too soon you run the risk of getting beat down by alot of people because you don't have the experience (aren't at that level yet). If you did ever progress further as a pro MMA fighter, having 5 losses from your first 8 fights or something isn't gonna be terribly good for your record/promotion of yourself.
Once you accept money for a fight its professional and you can never go back to amateur, and your pro record won't change. Think about that.