Have you already got it hooked up and everything?
If not, when you do get everything going, your PS3 will ask if you want to display in HD, and will find the best possible resolution for your monitor/TV. but* because it's a used system it may not do it itself, so you have to restart the video settings, which you can find out by searching google a bit.
next you'll have to go to your audio settings and select the W/R audio cables as your audio output, and it should be good to go.
*audio and video setting can be found under "settings" on the XMB menu.
I'll look for a link with a description of how to reset your video settings, otherwise you may just get a blank screen.
here's a good description, I guess finding a link is unnecessary.
Make sure the system is off.
Hold down the Power Button for 5-6 seconds - until you hear the second beep.
That's it, your video settings are reset and you should be able to choose your input and configure it from there.
You probably won't have to do an of this if the previous owner used HDMI, but it doesn't hurt to know. i once went from HDMI to composite for my PS3 and couldn't figure out why I was just getting a blank screen, until I remembered I had it set to display in HD.
you should also know that there are vents along all sides of the PS3, and if you live in a dusty or smokers house, these vents NEED to be cleaned regularly to make sure they don't get clogged with thick dust(mine did). it will overheat and destroy your system.
another good thing to know: google "safe mode ps3 forums" and read about how to go into safe mode, and you may want to "restore file system" to get rid of any corrupt files. this won't completely reset the ps3 to factory settings, but it will get rid of corrupt or misplaced files on the HDD.
if you wish, you can also restore the PS3 to it's original state through safe mode.
and if you have a neighbour with a PS3, beware of controller borrowing!!! I lent my neighbour a controller, got it back and tried using the PS button to turn on my system, and instead turned on his, lol.... I blindly used the controller from my house to turn off
his system, and somehow managed it. it's best to avoid it if possible, though.