Every 2 weeks you should be paid for the hours you worked in a 2 week period. Your company might issue paychecks one or two weeks after the end of the pay period. If you've been working 3 weeks you deserve an explanation for why you haven't gotten a check.
Many companies hold back the first paycheck. When you leave the job, then you will get that paycheck on the next payday after you have left. I think they hold it back in case you end up owing them money, like if it was proven that you stole something or deliberately damaged something etc,. If that's the case, you should start getting a paycheck on the next payday. But of course ask somebody about it, if they hold back the first paycheck when you first start working there.
Simple ask manager for real info as it varies greatly
you just walk up to the manager and say where is my paycheck?