Don't cancel your older credit cards as that can negatively affect your credit score.  Listen, cancelling ANY cards you have could do some/little harm if you have a history of good payments with those companies (Its like you lose that entire good history with that card once you cancel the card which can affect your score).  What I'd advise is for you not to apply to any more cards but rather just spend a few dollars regularly on each card and pay off the balance in FULL at the end of each cycle.  This shouldn't be too hard considering that you can schedule online payments for free these days from almost every bank now.  Also, try not to keep your credit cards from rotting on your report.  Cancel those which have never been used and those you used for the last time atleast 2 years ago. 

And finally, monitor your score as in the US, you need to look at yourself as an investment (eg: a Bond).  The greater your rating the more you could end up borrowing (for a house, car, business, etc.).  Start building your credit positively and it could help you a lot tomorrow.