^ You'll build up better credit if you can pay off your whole card each month. I've done that since I got my credit card (2001) and last year when I got my credit checked at the dealership it was over 700, which I think is almost as good as you can get it. I've heard that 800 is just impossible. I figure if you have the ability to pay it all off each month, do it.
Actually, 800 is very possible. Both my wife and I have a FICO score >800.
The advice given to the OP about opening a store charge card (Penney's, et al) is valid. Typically, they have less stringent guidelines and will offer credit to people with little or no existing credit.
The key here is to get in the system. Unless your gym reports to one of the three major credit bureaus (which is unlikely), your membership payment is not helping your credit score any.
Get a credit bureau report on yourself and see what is there. Get your FICO score. If you are <750 or so, you will likely not get in with Cap One or Amex. As another poster said, it is very hard to establish credit right now - money is uber tight.
Once you get a card, MagicChic is right - leave a small balance on it and let it revolve a little bit while making payments >minimum. That will increase your score.
One last FYI, choose where you apply carefully. Every inquiry to your credit file has the potential of lowering your FICO score.
Good luck.