Here are some tips to help you order the correct size. If you don't have a measuring tape, use a piece of string and hold it alongside a ruler.
Waist - Measure around natural waistline. This is the narrowest part of the torso, usually near the belly button.
Inseam - Standing comfortable with legs slightly apart, measure from just below the crotch to the bottom of the ankle.
Pant sizes are written WAISTxINSEAM. For example, 34x36 would have a 34" waist and a 36" inseam.