This Knitty article
has an excellent tutorial on how to pick up stitches. I will say though, that her photos show how to do it with a knitting needle. I am apparently not coordinated enough to do that, and I always use a crochet hook to grab the yarn and pull it through and put it on the needle. It's the same concept as with a knitting needle; you're just using a different tool.
Picking up stitches isn't that hard. What I've found is more difficult is picking them up evenly
. I've learned that for me the best way to do this is to figure out intervals and pick up a certain number of stitches in between each section. So, for example, say I have to pick up 50 stitches across a section of knitting that is 10 inches long. I would measure off one inch sections and tie in a scrap of waste yarn or use a split ring marker to mark off one inch sections. I would then pick up 5 stitches in each of these one inch sections instead of thinking of it as picking up 50 stitches along the whole ten inch section. This makes it much easier to space the stitches out evenly. Before I started doing this, I would tend to have them bunched up at one end or the other.
Another thing to keep in mind about that is--sometimes the pattern will tell you to pick up a specific number of stitches. However, if you need to pick up fewer or more to make it look nice, you can do that. Just make sure you pick up enough to keep in whatever stitch pattern you are doing.
By the way, if you have sewed both shoulder seams, it sounds like you will need to knit your collar in the round. That won't change the how to of picking up stitches, it just means you'd need a circular needle of the right size for your collar (usually 16").
If you don't have a circular needle, you can undo one of your shoulder seams, and pick them up on a straight needle around the neckline. Then you can seam the collar and the shoulder after you are done knitting.
Sorry for the novel, but hope this helps.