it depends on how much experience you've had, and how old you are.
and, of course, if you have enough talent.
first, i would suggest joining a drama group like stagecoach, or the playhouse
try and take part in school plays as well
keep on practising, and then look on the internet or in the papers for any auditions you might want to go to, then see if you can get there.
if you DO find an audition, then be prepared in case they reject you- the acting buiness is very hard, and 9 times out of 10 you won't get a part. but keep on trying, and don't give up.
if your really determined, you should get there.