A few things to make your shopping experience as smooth as possible:

Measurements are taken laying flat, the best way to compare measurements is to use your favorite t-shirt, jeans, etc. to compare sizes. 

For t-shirts, measure Pit to Pit (chest) and Tag to Tail (length)

For bottoms, measure top band (waist) double for waist size, from button to crotch seam (rise), from crotch seam to cuff (inseam), 8 inches down from top measure across (hips) double for hip size, across the leg (thigh), cuff (leg opening). Vintage denim is sized differently than modern denim so it is best to get familiar with your measurements!