I'd stick with a check card your first year of college. It gets you used to using plastic but doesn't have the temptation of a credit card. Always remember that getting a credit card is not in any way getting money. Try to think of it as another way to spend the money you already have. With a credit card you are only postponing the inevitable. (And sometimes making it a lot worse!)

As for bike purchases, I stick to only buying stuff if I need it to ride because something broke, wore out, or is causing me extreme discomfort. Don't bother with trying to sort out if something is a need or luxury. If you are currently riding without it then it must be a luxury huh? Once you acknowledge that 99% of bike stuff is a luxury then your natural thrifty nature will take over and let you weigh the cost/benefits of every purchase.

You mention paying off necessities and then using the "spare cash" for bike purchases. "Spare cash" is a silly idea that's only good for easing your conscience. All the money you have is the same; every dollar can be either saved or spent and it's up to you to make that choice. Everyone has a level of luxury that they feel comfortable sustaining. Once you decide on where to draw the line (which'll probably change based on your circumstances) don't feel guilty about your spending, but never think that you "need" or deserve all that "spare cash".

This might seem to be a hard-line but I've found it really helps me to keep my mind off my money and my butt on the bike.