I know that people come to acting from a number of difference socio-economic levels, but let’s just specify middle class Joe/Josephine Wannabeactor.
If you got a notion at 21 that you wanted to be a TV or movie actor what are the next logical steps to make that dream come true?
(Are you 12 or 21?)
Move to Orlando, LA, NYC. Otherwise, you’re just handicapping yourself.
Find an agent.
Act in anything that will have you act - get some experience/exposure.
Fix those teeth! 
Be really good-looking.
Sorry… age 21
This is the best step one, for sure.
At 12, hope Disney finds you. Selena Gomez and Demi Lovato were both picked by them at a talent search in their home state of Texas, and now look at 'em. (Don’t know who they are? Ask someone who is 12- they’ll know.)
At 21…I have no idea.
Be willing to go on auditions for anything without getting hired. Don’t get discouraged.
Recognize that you are going to have to do something else until you get hired. Very few people get their big break right off. It’s probably going to take years.
Be willing to take any role you can get.
It helps to take training, even attend drama classes in college. Take courses at theatre companies. It won’t make anyone hire you, but you make contacts and you do learn.
Network, network, network, network.
Look your best, get your teeth fixed, dress well.
Network. Network some more.
Get proper photos from a photographer who specializes in publicity shots.
Network some more.
And yeah - take any and all jobs short of hard core porno.