hi...you can use ET Geowizards toolbox...and find the polygon toolset....if you dont have that toolbox..please search here in forum....just follow the step of that toolset...easy use....enjoy
@HaNoi ....you can use vegetation indexes such as NDVI;RVI:MSAVI2 or band ratio to make easy decision for densed forest area or just medium forest area.....you can use ENVI or ArcGIS for that work
[email protected] can try this link http://earthexplorer.usgs.gov/ and http://glcf.umiacs.umd.edu/data/ for free downloading of the satellite image....please hurry up......he.he..he