Before purchasing one you will need to make sure it is compatible with your computer. Modems - A modem is the method by which your computer connects you to the internet. Many newer machines have modem's built in. The slowest modems that are generally in use now are 14k. There are also 28.8k, 33.6k and 56k.
External modems are easy to install, they just plug in.
However, the configurations of your machine will have to be modified to accept the modem. If you can't do it yourself, call on your friendly neighborhood techies to help. If you upgrade your internal modem you will probably need someone to help you do that. If you want a 28.8 ask around. Someone who upgraded probably has one sitting around that you can talk them out of.