Hi Miss B,
You're experiencing two different problems. If you are reading that you have a small amount of memory, remember there are two kinds of "memory". What you are talking about is the "hard drive"...that device that holds all the programs, the operation system (Win 98, XP, Apple, etc). You can usually buy a larger hard drive and install it if you're computer can handle it. If you choose this option, move most of the "archive files" (files such as pictures, movies, text files, spreadsheets, etc) to the new drive to free up space on the old one. This option is also available by buying an "external hard drive" which is exactly what it sounds like, a large storage drive that can hold an addtional operating system and extra files. It connects through a "USB wire and port", that's a slim headed plug that fits into a slot on the computer.
You need to deal with the other kind of "memory" the "RAM" or "Random Access Memory" , these are small strips of electronic connectors that fit inside the computer. It will allow your computer to optimize its performance by increasing its "horsepower." This will also allow you to add a CD-writer. Once again, it may be more cost effective to buy an external device by the time you factor needing to have the device installed. Some stores offer free or lower price installation if you buy the device from them. Beware, however, that many of the big box retailers may charge as much as a 40$ "bench" charge just to open up the box.
Increasing the RAM by adding "SIMMS" of ram is fairly easy to do. You can find step by step instructions by google-ing "Instructions for instaling RAM". Hope this helps.