To learn the pipeline world which is a whole universe unto itself for a welder....different lingo, practices and procedures. Go to the places already listed....find a pipeline welder at a gas station or getting breakfast in the morning near some work(you can tell cause his bed is probably unpainted and very short....not much tools).   Put on your best personality and beg him to be his helper...this will be difficult since his current helper is riding with him.  If you can get the gig then you can really see what the pipeline world is like....not for everyone.  And of course morally you you should be ready to see that helper you replaced sitting at a gas station 600 miles from home and it cannot bother you much.

Ok in all seriousness it is a closed society for hands driving thier own rigs (trucks with lincoln welders on them and occasionally rarely pipe pro millers).  It is not easy to get into, your not gonna fill out a job application and go out on the pipeline.   The best way to see it, get a feel for it and get a "CHANCE" to get started is be a helper on a pipeliners truck.  In other words be his slave, grind everything, brush everything, roll out his leads and tools and roll them up bring him coffee and breakfast in the morning when you show up, be first to jump out at the fuel tank and fuel that machine and truck.  Do a great job and he may let you practice welding off the back of his truck at the hotel or camper and give you a thumbs up to the qc to take a test at some point.  If you want to just beat the door down calling companies you will get a sore fist quickly....you need to prove yourself and know someone to get a chance.
It is not too hard...just a different world. 

Its not a bad way to make a living at all, lots of perks, lots of travel.  Your gonna have to dig if you want to get in it and stay busy.