When you get your yarn you will need to do a gauge swatch in Stockinet stitch. I like to use 20 stitches/20 rows. Really look at your fabric and be sure you are happy with the density of it. If not change your needles and do another one.
Note your gauge then wash and dry the swatch as you will be washing the sweater. This washed sample is the gauge we will base your measurements on. If it is different than the unwashed gauge you will want to take that into account as you are knitting the sweater and trying it on. You may find that you need to knit it a bit longer to make up for any shrinkage. You may omit this if you will be dry cleaning your sweater.