Usually telephone lines are the red and green wires for the signal. In older homes you'll find the black and yellow wires which were used as a power source from the central office. This system is no longer used anymore so you can do as you wish with them.
Check your telco box outdoors (usually there's a standard screw on the user-accessible section) and check which wires in your house are being used. It shouldn't hurt to use the unused wires for an ethernet connection. You'll need two of the twisted pairs for it to work though. Usually it's the green/green-striped and orange/orange-striped pairs.
Then, you'll need some connectors and a Cat5 crimping tool (these things cost $40 so borrow one if you can. Put the wires in the proper pins (guide:
here). All the one's that say N/A don't hook up. You only need those four for an ethernet connection to work.
The negative side is, you're not going to be able to hook more than two computers up to such a network. In the page above, use the wiring diagram for B on one crimp, and A for the other crimp.