First, you have to find out what kind of RAM that your motherboard uses. Find the spec sheet for the exact model of the computer you use. You don’t have to understand what the type actually means. All you need is the type. Also you need how much it came with when you bought it and how much it can expand to.
You now have to open up your computer to see the computers “guts”. On the motherboardis where the RAM stick(s) will be located. Find out how many slots that can be filled with RAM.
Go online to Newegg or another store that sells computer parts. Find the exact type of RAM that your computer uses. Buy the middle of the road when it comes to price. Too cheap means it will probably fail fairly easily. The most expensive is overkill for most uses.
Save the old RAM that you removed just in case your new RAM fails. That way you won’t have to wait to use your computer while you order and wait to receive replacements. Congradulations. You have just upgraded your computer!