Is there a website which can give steps to open up a computer?

What do you mean by "open up"?
Starting or (dis)assembling?I could try to help you if you can give more details...

Its not a stupid question but if you're asking that question you shouldn't be opening it. I'm an Information systems technician and wouldn't recommend opening it if you don't know what you're doing. If you still insist PLEASE WEAR LATEX GLOVES TO PREVENT ESD. Electrostatic Discharge is when static electricity from your body chooses to discharge itself. 1 volt of static electricity is enough to kill some parts in your computer. Static electricity comes from clothes and other things; for this reason DON'T WORK ON YOUR COMPUTER IN A CARPETED ROOM. If your computer has a power switch on the power supply on the back, leave the computer plugged in but turn that switch off. If there is no hard switch, unplug the computer. If you leave the computer plugged up, in case of accidental ESD, the static discharge will go to the power supply and into the grounding of your house, NOT your computer.

I only suggest adding pci or Agp cards for novice people. DON'T MESS WITH THE PROCESSOR, HEAT SINK, RAM, OR TOUCH THE IDE CONTROLLER ON THE BOTTOM OF YOUR HDD.

Look on the back of the computer on the right hand side (right side when back of CPU is facing you) Look for the seem for the side panel, there should be two screws, one near the top and one near the bottom. This is a generic case. Depending on what brand of computer you have, each opens different. Dells open up like a suitcase. Hp computers use to have one whole cover that slid off to get "bare bones". please e-mail me if you have any doubts or questions before you mess it up.
hope this help............