What a great question to ask. You can become self employed today working in the fields of Advertising and Promotions. I would encourage you to look at various Colleges / Universities where you can start building foundations for the field. A second piece to the puzzle would be to network professionals in this field on "LinkedIn.com" or find groups that are in your local area where advertising professionals "meet up." Lastly, try your hand at working for an Advertising Agency and or the Media Outlet i.e. TV/ Radio. These types of organizations will train you further and help drive those building blocks to create your own business in the field of Advertising and Promotions. One more thing, I'd encourage is find a mentor to talk with in the field and ask them for direction or to just be a sounding board. I hope this helped you!
Here are some statistics from the United States Department of Labor. https://www.bls.gov/ooh/management/advertising-promotions-and-marketing-managers.htm
Absolutely. Advertising is not going away anytime soon, although it is a highly competitive industry.
I started my Advertising company in 2015 and we grew to a team of 11 within 18months - there is generally lots of demand. Every company needs to sell &/or market themselves. I have sold this business since. If you would like to start your own advertising company, my advice is to find your niche (could be industry/vertical) and specialize your offering.