To get a fewer number of stitches, you need to go up in needle size (the stitches get bigger, so you get fewer per inch).
Does the pattern tell you what to do the gauge in? It will usually say the gauge done in ss (stockinette). Check that. I think that is pretty typical.
I think that if you're doing a sweater, the most important gauge to get correct is the horizontal and not the vertical (# of rows). You can easily adjust the length of the sweater by doing fewer or extra rows, depending on the need. Knitting with the wrong gauge across will make the sweater too big or too small.