Here is a site showing how to use the circular needle.
What I suggest is that you put a marker between 1st stitch
and last stitch so you know where to stop!
http://knitting.about.com/od/knittingskills/qt/circularbasics.htm
Hope this helps and good luck.

It also depends on what you're making with circular needles. I LOVE circular needles for EVERYTHING. You can knit flat stuff with circular, not just round stuff (like SOCKS, my addiction!). I like that the needle (stiff part) isn't as long. WHen starting I liked 8,9 or 10 as it wasn't too big nor too small. AND I liked bamboo, and about a 24 inch cable. But it's something you start aquiring, and depends on what you're knitting.

Vickey-MN

Circular needles can also be used like straight ones, where you just go back and forth. Your pattern will tell you the size needles you need. I'm doing an afghan with 36" long size 8 needles. I couldn't use them on a small project.

I like to use circular needles because when I have to stop for any reason, I just pull up both needles and shove the work onto the cord. No chance of stitches falling off.

I like them so much, that I bought a whole set of needles with interchangeable cords. Of course, the first thing I made after that purchase required a size/length that I didn't have!! Oh well.

Find something you would enjoy making, get the needles that the pattern recommends, and have fun. In a few years, you will have quite an assortment of needles.

-Babka