I've always had a problem with fishing wire slipping at the crimp over time so eventually the clasp and the beads just explode off
I use tiger tail jewelry wire now because its coated and just has proven to work very well for me. I am rough on my jewelry and I have yet to have any crimps slip or the wire snap. It is also nice because its coated and not very sharp and the end you cut at, so if you end up with a piece sticking out for any reason at the clasp, it shouldn't stab you. I used to have that problem alot when I first started making jewelry.
As far as making the necklace you really just need to measure how how much wire you want (which will differ on who or what its for). Find a good length. I always clasp off one end so I have a stopper for the beads. Make sure you know where the center of your necklace is going to be then add what you want in a pattern towards the center, then repeat it going down the other side. I like to leave at least an inch of room extra at the end for when you close off the strand, that way it doesn't slip out of your hands easy. Many times did I not give myself enough room and I ended up with a lap full of beads >.<.
I hope my explanation made sense...I tend to ramble and in the end no one knows what I am talking about haha...I can make a short picture tuturiol if you guys want