Let me take a shot at this.
In order to learn the craft and business of any photography discipline, you would need to apprentice yourself to a working photographer and this is called being a photo assistant. If your interest lies in the world of advertising, you would have to head to the big advertising markets, New York, Chicago, LA or possibly Dallas or Atlanta and look for work as a 'freelance' photo assistant (which is day by day work, hired for the length of the job, one day or two or a week). It would be great to have an 'in', like knowing a working freelance assistant or photographer, but you can do it cold as well. Most of the pro photo labs have bulletin boards that photographers and assistant can post opportunities and availability. As a freelance photo assistant you can work for quite a few different photographers, learning about their styles. You would learn more that way and after a year or so, you might look for or get offered a full time assistant position with a photographer. After a few years, you might move on to another photographer and work there for a year before going out on your own.
Before digital took over, there were quite a few catalog houses in major markets, that employed 6 or more photographers full time to photograph catalogs year round. It was easier to get starting work there and learn the craft, but you only took home a weekly paycheck. These places have disappeared. If you do a web search for catalog photography studios, nothing turns up like that, only individual photographers that will do catalog work. The only good news is that most photographers that have work will need an assistant, either full time or freelance. There could be quite a few photographers in even small markets that would need assistants. Write or email them with your interests and availability, you could get lucky.
Not matter what way you go, the one thing you will need is a portfolio of your work. It is how you will be judged for a job. Some have gone to commercial photography schools to get a starting portfolio. In my opinion, not the best bang for the buck, as most of the portfolios look so similar, they become boring very quickly. They give out the same assignments year after year, do a jewelry photo, do a perfume photo, etc. and with the schools lights and equipment, the photos all look the same. You might have better luck at state type colleges that have photography courses, even though most colleges are geared to the 'art' side of photography. You can produce a portfolio on your own. Find an image that you like and try to reproduce it. If there is a ladies hair product photo that you admire, go to the nearest drugstore chain and buy a different product, use a different color or background, but try to equal the quality of the other photo. You won't have all the great equipment that established photographers will have but see what you can do. If it looks like road kill when you are finished, you have a long way to go. If it looks pretty decent. put it in your portfolio and do something else. You don't have to buy brand name expensive stuff, knock offs will work fine. On Canal St. in NYC, you can buy fake rolex watches for $60, fake Coach bags for $100 and fake diamond rings for $25. Stores like Target have plenty of visually exiting merchandise that is inexpensive, be creative. Talent can go a long way.
Hope this helps, just remember that I can't guarantee all of the above info in this day and age, with the economy the way it is.
Brian
