You will have to give us your system specs. Every system is different and can use different ram types.
What motherboard do you have? When you find that out, go to the manufacturers website and find out what ram type you need. DDR2? DDR3? It will say on the website.
Once you find that out, figure out how many ram slots your motherboard has. Probably one or two. Buy a four GB stick or two 2 GB sticks. Unscrew your case and you will see a 2 GB stick of ram already in there. Put in your new stick(s).
Obviously your machine needs to be off when you install your ram sticks. It is very easy and takes just a minute. The only thing we can tell you is to find out your specific system specs and what type of ram your system can use. Then you buy that and install it.
For an installation video, check out youtube. There are a million of them and they will show you exactly what you're looking for. The process is very very easy. I can change my ram sticks in under 1 minute so don't be afraid to get into your system.