I was so confused when I went to go measure for the first time too! I looked it up on the internet and these are what I use. As long as it's consistent it shouldn't matter:

Bust: ~ the nipple line
Chest: directly under the boobs (I'm assuming you are a woman-- excuse me if I'm wrong!)
Waist: ~ the smallest part of your "waist area"
Bellybutton: obvious
Hips: ~ the widest part of your hips
Thighs: ~ the widest part of your thigh
Knee: just above the knee cap
Upper arm: ~ the widest part
Forearm: ~ the widest part

Have someone help you if you can so they can make sure the tape measure is in a straight line all the way around your body. I would encourage you to measure-- you'd be shocked at your findings. I'm easily seeing big differences on a weekly basis already!

It is actually recommended for women to measure the same time of day each month (same time during cycle too) to get the most accurate results.