Yeah, DF is pretty rough on newbies, but much MUCH nicer than many games, especially because of the forum community and wiki(which you should definitely spend some time with).
First of all, you're talking about a magma pipe, why are you talking about water? Looking at this thread as closely as I can, I just can't figure out what you're getting at with that.
On to handling magma, you should dig tunnels going to where you want the magma to be(ie, under where you intend to put the magma buildings). You're implying that you know what you're doing with this. Make it close by the pipe just so it doesn't take an eternity for the magma to get where you want it. You probably want to put in a magma-proof fortification or something to block fire imps. Or prepare traps to catch them(advanced option, don't do it unless you're confident). That last wall between your tunnels and the pipe, channel out from above and then seal off. You're pretty well set at that point.
See the various tutorials around. The tutorials at the top of this forum. They both cover magma. Look for tutorials/advice/information on the wiki, as well. Specifically, look at
magma and
workshops. There are actually plenty of resources out there to help you. Take a look and they'll turn up...or wait twenty minutes. The super-experts are pretty active. One of them ought to turn up and help if you're having trouble with anything I said.