Originally Posted by Hardrock23

https://www.google.com/search?q=how+...ih=887#imgrc=_

Look at those pictures...
You want to measure from the center of the bottom bracket (the middle of the crank arm, basically) to the top of the seat tube - for the seat tube measurement.

For the top tube measurement - Measure from the center of the seat tube to the center of the headtube.

For the stand over measurement - measure from the ground to the top of the toptube, near the middle section.

Your bike looks like maybe 15-15.5, but its hard to tell since its not lined up correctly.
Below are some pics from the search. You can also watch youtube videos (just search "how to measure a bike frame").