Welcome on the forum, Min
You are right, masking is a very important resource for Elements!
Elements offers masks in adjustment layers and solid color layers. Unfortunately, not so for common layers, so that you'll have either to install free add-ons for masks in normal layers, or learn the way to use an adjustment layer mask as a work-around.
Using a mask is simple in theory and practice: If you paint black in the mask, you are hiding those parts; with white, the parts show through; with grey they show with reduced opacity...
Do a search in the tutorials or resources sections of this forum. Other members will certainly give you links to tutorials; anyway please ask any question about this!
Edit:
Here is one:
http://coffeeteaphotography.blogspot.co ... orial.html