This topic has been covered dozens of times on this site and others. I suggest a search on "gunsmith" and related terms. IMHO, you can't learn gunsmithing from a video course; you need hands on the machinery. (Would you want to have your appendix removed by a doctor who learned from a video course?)
One thing you can do at low cost is to see if a local community college has a machine shop course (the one in this area has an excellent one).
Some folks will tell you that all you need to become a gunsmith is a screwdriver, a mill bastard file, and an FFL. Alas, that kind of thinking has resulted in a lot of ruined guns and a lot of very unhappy people.
Frankly, unless you are willing to invest a lot of time and money to learn the job, and then go into business right, I strongly recommend you not work on other people's guns. One of the hardest things to do is to make a business out of your hobby; unless you have a lot of dedication, it is almost a sure way to go broke, and maybe be sued, have legal problems, etc., as well.
Jim