Posted by Nathaniel Ryckman on URL: http://imagej.273.s1.nabble.com/Basic-Image-J-Help-tp3685011p3685015.html
The general process for detecting regions of interest (ROI's) is:
Threshold or Edge Detection -> Binarize -> Watershed/Fill/Etc -> Particle Analysis -> Measure
I personally think that this site has a good tutorial that gives you thorough examples without getting bogged down in detail:
http://www.macbiophotonics.ca/imagej/
Here's an e-mail that contains websites with more detailed documentation that I sent one of my co-workers who was interested in imageJ: