How can a 12 year old boy start acting on shows or films?

-How would they go around it?By getting into a good acting class and working really hard for a few years, while getting experience doing stage plays, etc.

Then you might be good enough to get an agent, if you're seriously talented, and the agent would then get you some auditions.

You'd then have to attend and win the auditions.

No-one employs untrained inexperienced beginners.
Only if your parents are onboard with it because you are a minor by law on set until you are 18 and one of your parents would need to be with you at all times.

If they are on board, you need to start with an agent. You can audition without one but its a lot easier if you have someone working for you.

BEWARE - AGENTS DO NOT CHARGE MONEY UP FRONT they take a percentage of your pay when you work.

Good luck!
heck yeah look at good luck charlie,but i don't know how to start,just get noticed
Talk to a producer
find an agent