Before we put the horse behind the cart, at this point you should have asscertained what kind of ram you currently have and have gone to a store that sells ram, optionally you can buy it online if you want to wait for the postal service.
You should be able to go into control panel> system and I believe it should be on the front page of that panel that comes up.
First remove the power cable from behind the box, remove your mouse and keyboard usb cables, disconnect your monitor and appraise the box part.
Depending on how old the computer is , its just a matter of laying the box part of the computer on its side, remove the screws that are on the edges at the back and either a side panel will come off , or the whole outer casing should.
Locate the ram modules , by first looking at the ones in your static bag or plastic casing , there will be usually two or more slots containing sticks of ram and the slots they go into will have levers on the end that you push down , to pop up the ram.
The installation of the new ram will be the reverse, align the ram stick the way the old one came out and push down firmly, the lever should pop back up.
Reverse every step and fire up the computer ,and your new ram should be recognized.
Knowing what kind of ram you have is important because if your computer is old enough, 486dx old , p1, p2 etc. the ram was different.
Hopefully this will have given you enough information to decide if this is what you want to do , on paper its easy and most of the time , it is ,but some people have had problems doing this and bad things have happened.
Declan