Unplug the computer from the electrical power first of all before working on the computer, and then touch the back so you do not have static electricity in you to make sure you will not wreck anything in the computer. The proper way to do that is to have a grounding strap hooked to the computer that has a finger attachment also on it, and you wrap that around you finger and hold it there because it is velco and sticks to itsself like a bandage.
If you are careful and make sure you touched like the back of your computer (not a board) on the metal part of the computer chassis, then you should be fine, unless you are moving around on carpet instead of standing still and causing yourself to pick up static electricity along the way (rubbing your feet perhaps on a carpet and so), and if you do not get a shock from your hand touching the back of the computer then you are okay, and if you do, then the static electricity you had in you will be dissipated when touching something metal (like the back of the computer that is not painted, but bare metal).