The first thing you need to ask yourself is if you want to invest a good amount of money and time. What a lot of people see with bitcoin mining is a get rich quick scheme, which I can tell you it is not. You'll need to invest in good hardware if you want to have a decent to good hashrate (higherhashrate = more coins), and you'll need to wait many days (45+ days) for your hardware to get you some coins, even on mining pools. Also, being good with computer hardware (as in maintenance, overclocking, and building computers) comes in very handy. If you want to invest your time and money, start looking at the how to get started topics all around the newbie and mining threads.
A fair warning, old PC's don't really mine well. Processor mining is overshadowed many times by GPU (graphics card using opencl) speed. If you want to be able to mine well, you'll need to invest in good graphic cards, like the ATI 5XXX and 6XXX serires (not mobile ones found in laptops).
I don't want to sound to deterring, but most newcomers asking these questions don't usually know what they're in for.