The link that Dave has put up is excellent, this gives both new and old wiring standards/color codes.
RJ-12= 6 conductor, 3 lines (3 pair), if you have more conductors (wires) than that, then you may have a UTP LAN cable/Ethernet Cable (UTP = Unshielded Twisted Pair, LAN = Local Area Network).
As for the installation, this is the same as installing any other home telephone line in the US or network jacks.
If this is for a digital PBX or data installation, make sure you mind the polarity of the connections. If it is for a 3 line voice only communication cable, polarity won't matter too much, but I will say, the quality will be better and the overall installation will be better if your colors line up correctly.
As far as the actual jack connection, it should be color coded, simple as that. If it is not color coded, or the colors don't match, make sure you have the correct jack for the job. Also note, there are several types of connections that could be present, there are punch down type jacks, and screw terminal type jacks, its up to you, I personally like punch down blocks better, they do need a special tool, which is cheap if it doesn't already come with a plastic one.
Post again if you have any issues, and have fun!
Matty-
