An internal modem will run you about $15. If you can use a screwdriver, you can install it yourself.
Buy the modem. Read the directions. Youll either install the drivers/software (which will come with it on a CD) before or after you put the card in your machine. Just do what it says. To put the card in: turn off the computer, take the cover off, put the modem in the slot securing it with the screw, put the cover back on, turn the computer on.
It's really not hard.