Suit measurements are critical if you're looking to get tailoring, alterations custom work done for your clothes. Its also a good idea to get measured so when you are out looking you know your size and can hop on some great deals.
One easy way you can get your measurements is from a men's store. They'll be able to do it, although they will try to pitch your their deals along with it. Get the numbers down on paper so you'll know them. Remember though that measurements can change over time especially in the chest and waist area so get new numbers periodically. Weightlifting can also change your shoulder and chest dimensions as well.
You can have a friend do it as well with any tailors measuring tape. You should also have a shirt, jacket, nice pants (not jeans), and dress shoes to help with some of the upper body numbers. Here are the pieces you'll need to get:
Chest: Take the tape around your upper torso at the nipple level. Do not flex your chest and make sure the tape is not uncomfortably tight.
Shoulders: Measure across from shoulder to shoulder.
Jacket Length: Measure from the jacket collar near the shoulder seam down to the jacket bottom. If you have a jacket you like you can wear that as a guide. If you're not sure, then a good rule is to end at the knuckles.
Sleeve: Take the tape from the shoulder seams connection to the end of the sleeve. You can also measure to the base of the thumb joint.
Waist: Wrap the tape around your body in the spot where your pants top will be. It should be a good fit but loose enough to allow an index finger of clearance. Keep in mind that it mind not jive exactly with what you have bought for pants or jeans in the past. Manufacturer measurements can be wrong!
Inside leg (inseam): Keep a tight tape from the pants' crotch and go down to the bottom of the pants. If you're not sure, measure to the middle distance between the dress shoe top and sole top.
Outside leg (outseam): Keep a tight tape and measure from the pants waist to the bottom of hte pants. Again, a good length would be between the top of the shoe to the sole.
Keep in mind that these are suit measurements, but its also a good idea to get shirt size information as well. Also, a custom suit will need more detailed information as it will be tailored specifically for your body.