New water supply lines are usually copper or plastic, but if you need to repair or extend a galvanized pipe system, it makes sense to use the same material.
Almost all gas lines are black threaded pipe, which is installed the same way as galvanized pipe.
Measure carefully, keeping in mind that 1/2- and 3/4-inch pipe goes about 1/2 inch into each fitting. Then have a home center or hardware store employee cut and thread pieces to these exact measurements.
A more flexible strategy is to estimate your pipe and fitting needs. Then you can buy long pipes as well as a variety of nipples -- short lengths of threaded pipe ranging from 1 to 12 inches. Also buy extra couplings. When you come close to the end of a run, you probably can create the correct length simply by combining nipples and couplings.
About an hour to cut into a line and install several fittings and pipes
Hacksaw or reciprocating saw, two pipe wrenches (14-inch wrenches are a good choice), groove-joint pliers
Careful measuring and planning, firm tightening using a pipe wrench
Shut off water. If there is a nearby union fitting, you may be able to avoid cutting a pipe.
Threaded pipe lengths and nipples, pipe-thread tape or pipe joint compound