You need to know what kind of memory is compatible with your computer. There are a few easy ways to find out...
1. Open your computer and take a look at what's already in it. You're always safe getting the same kind of memory, just more of it.
2. Go here and use the crucial memory adviser. It will tell you exactly what kind of upgrades you can use. http://crucial.com/
3. Post the manufacturers name and model of your computer here and someone will be able to tell you what kind of memory you will need.
4. Check out your computer manufactures web site and go to support. Look for the specs about your computer and find the type of RAM memory that is installed.
Personally I'd just go the Crucial.com (no. 2 above). That's the easiest and fastest.
Installing memory is one of the easiest upgrades you can do to a computer. It will be slightly different depending upon if you're using a laptop or a desktop.
Laptop computers generally have a small cover on the bottom you have to take off to access the memory. Once that's off you just pull the clips that are holding the memory in place outwards and the memory will tilt out. Then you can pull it straight out. To put the new memory in, do just the opposite. Push the memory down into the slot, then tilt it back in until you hear the clips "click" into position.
Desktop computers usually have two or four slots for memory, configured in one "bank" of two "slots." This isn't always the case but it's the most typical. To install new memory just look at the memory stick and the slot it's going to installed into. There is a notch in the memory module that lines up with a notch in the memory slot. (This is so the memory can only go in one way.) Line up the memory module in the slot and push it down until the side clips that hold the module "click" into place. (There are also little slots in the side of the memory stick for the clips.) Do not force it if it's not popping in the slot; you may break it. Look to make sure you are putting the memory in the correct way and be careful to push the module in straight.
The type of memory your computer needs will determine whether you need to buy two matching sticks or if one will be enough. The memory configurator at Crucial.com will tell you that.
If you have empty memory slots available you can just add more sticks of memory. If you don't, you'll have to toss the memory you have and replace it with new sticks.
There are plenty of sites on the Internet with good tutorials about how to install memory. Just do a Google search and you should get lots of help. Good luck!