What does your pattern say? If the sweater is worked flat in pieces (knit the back, then the front, then the sleeves), then you will be working back and forth (not in the round). You are probably using a circular needle because you would have too many sts for straight needles.
If your pattern starts out with CO x number of sts, JOIN for working in the round, then you will need to alternate knit and purl rows for garter stitch.
Are you working on a free pattern? If so, you can provide a link (or at least the name) for further clarification. You have come to the right place for help!