What do I need to know when replacing the processor in my computer?

I am thinking of upgrading my processor in my computer, and i am just wondering what sort of information i need to know before doing all this.
Like...is the motherboard compatible with the processor. etc.
Update: @MaskedMusketeer
BIOS update? Is that...hard/easy to do?

Also, I have absolutely no idea where the best answer thing is.
Update 2: Right, thanks everyone for your help, i figured out that both the processor and the motherboard are so old and rubbish its not worth replacing :/
Thanks anyway :D
8 answers 8