Watch brainpopjr first (for beginners) - Sink or Float under the science tab.
http://www.brainpopjr.com/science/forces/sinkorfloat/preview.weml
Then click on brainpop and watch buoyancy under the science tab.
http://www.pbs.org/wgbh/nova/lasalle/buoybasics.html
Read the text to help understand the concept of buoyancy