There are some great sites for learning how to measure your body. It is best to do this with a friend who can help you hold the measuring tape around your body. If you do not have a close friend nearby to help you. Remember you have a closet filled with clothes that fit the way you like.
Take the measurements of your clothes and then add 2” for ease and seam allowance.
This technique can also help you choose the correct template size to cut out for a new sewing pattern.
Here is a cute worksheet you can print and write down the measurements. It is nice to keep these over the years to see their growth.
This worksheet by Keyka Lou Patterns is so wonderful.