You could try this page with a simple explanation of electron dot diagrams:
http://www.fordhamprep.com/gcurran/sho/sho/lessons/lesson38.htm
In my words, these digrams are a respresentation of how many electrons are located in the outermost(valence) shell of the element.
I'm not sure if you know the structure of each element, it's made of valence shells numbered 1,2,3etc... which each contain electrons in orbit(s) s,p,d,f,etc...
You can actually use electronic configuration tables which list the arrangement for each element in the periodic table,eg. Sulfur's is:
1s2 2s2 2p6 3s2 3p4
As you can see, sulfur has 3 shells. The third is the outer/valence shell. The superscript on each orbit (the letters s,p,d,f, etc..) indicate the number of electrons. So, you simply add those superscripts associated with the 3rd shell, i.e. 2 + 4 = 6, and you get the electron dot diagram showing six dots, as on the webpage listed.
Note: There is a maximum of eight electrons in a shell.