One of the toughest things about making sweaters is figuring out the construction and shaping. Unlike a blanket or scarf, it's not just a matter of casting on, working for a certain number of inches and binding off. There are angles and curves to be considered There is a somewhat perplexing instruction that is very common in pullover patterns, wherein you work some stitches, join a new ball of yarn, bind off some stitches, and work to the end of the row. Now you've got two balls of yarn going--one for each shoulder--and you need to work both shoulders at once to make sure they come out even. It's one of those things that's a little confusing when you read it, but when you start working it you'll find that it goes pretty smoothly.
Here's a breakdown of the steps:
Work across the indicated number of stitches with the first ball. Add the new ball in the same way you would when you run out and add a new ball (work the first stitch, leaving a long tail you will weave in later). Bind off the indicated stitches and then work across the remaining stitches with the second ball.
You now have two balls of yarn attached, one on each side of the neck.
Now you will work both sides at the same time, ensuring you have the same number of rows. Work across the row with the yarn on that side to the end of that section. Drop the yarn. Pick up the other ball and complete the row on the other side of the neck. And you just continue on like this for the indicated number of rows or inches, working any decreases that might be indicated as well.
You do need to be careful of a couple of things when you're doing this. First, you need to make sure you don't put the work down after working one shoulder but before working the other, because it's hard to tell when you pick it up again which shoulder you ended on. If you absolutely must put it down, work at least the first couple of stitches on the next shoulder so you can be sure to start off in the right direction.
You also want to remember a couple of things when you're binding off: First, be sure not to bind off the last stitch of the first shoulder. To start the bind off, knit two and slip the first stitch over the second stitch -- don't knit one and slip the last stitch worked for the first shoulder over at. Conversely, when you get to the other shoulder, you use the first stitch of the shoulder to bind off the last stitch. So you'll work the last stitch to be bound off, then the first stitch of the second shoulder, and slip the last bound off stitch over the first shoulder stitch. Now that first shoulder stitch will be already worked and on your right needle -- just keep working across from there.
Finally, any decreases that you may need to do will usually just say something like "at each neck edge". That will be at the end of the first shoulder worked across and at the beginning of the second shoulder worked across -- the edge of each shoulder closest to that bound off section. The "armhole edge" on the other hand will be the edge furthest from that bound off section -- the beginning of the first shoulder worked and the end of the second shoulder.
As I said before, this is one of those things that seems like it's going to be really confusing when you're reading through the instructions, but once you sit down and get started, it will all fall into place.