Frequency of motion in a frame

I have recorded a video of a person breathing and the files I have obtained are a series of images from a camera at 30fps and I want to measure the breathing rate in this. I am planning to do so by implementing optical flow algorithm on the ROI(chest area) of the frames. The problems I am facing with proceeding are : 1. How do I check all the individual frames? 2. How do I define the ROI? 3. How do I track a single point in this ROI?

Since I am beginner at MATLAB, I am expecting a rather detailed answers. Thanks in advance!