Need some help installing OS 10.5

Hey everyone,

I just recently purchased my first Mac, it's a Macbook with a 2.2ghz Intel core 2 duo, 2gb of ram, but only a 13" screen, lol. I'm use to working on PC's, so learning how to use a Mac is exciting for me cause it's a something new to learn! Anyways, I bought it second hand, but it came with no discs. The previous owner has alot of files/programs on there which i want to remove. I'd rather format the HD and reinstall OS 10.5, but I don't have the disks. Is there a way to repair or reinstall the OS without actually deleting it, so that it's a fresh copy registered to me with all the other owners stuff deleted?

Also if I end up having to buy the software then I will, but then how do you install the OS, like a PC you just set the BIOS to boot from CD first, then press any key to boot from CD..I noticed when my Mac boots up (really fast too), I couldn't see anythign that would let me get into the BIOS...or does a Mac even use BIOS, isn't it EFI or somethign? I don't know much about Mac's, but I do know enough about PC's...not sure if the model i got is crappy or a good one, but any help would be appreciated.