Actually, your house is probably wired for up to 4 lines coming in the house. 4 wires=4 lines. But that is up to the phone company to set up as everything up to your house is their responsibility and they have to set up a phone number and make changes at the central station for you to have additional lines.
Now as to adding outlets to the one line you apparently have active now, this is really easy. Just go to Radio Shack and you will find a large assortment of plugs and jacks that will allow you to add as many outlets in your house as you would like. Now some words of caution. You can still only have a connection with one person outside at a time, since you only have one line going out. 2 people (or more) can pick up another phone while you are already talking to someone outside, but will only be able to talk to that person outside and the quality of service will be degraded greatly with each additional extension that is active. If you have a computer modem on that line, nobody else can call in or out when the modem is in use. And Do Not get Call Waiting on your computer line. If you do, each time someone calls in, you will get a beep on your line that will disconnect your modem and you will not know why. You will have to reopen your connection to the computer server, and then the real live person that was trying to get you will call back and you will have to start again. And one other thing, if you have or expect to have teenagers, never admit to them that you even have a phone or you will never get to use it again. :o)