Period: http://www.englishclub.com/writing/punctuation-full-stop-period.htm Comma: http://www.englishclub.com/writing/punctuation-comma.htm

Those resources are good ones. I also find that many new English speakers tend to write sentences that are far too long. Normally there can only be two or three clauses in an English sentence before you should use a "." and start a new sentence. Hope that helps!

The really important thing is this: no space before the mark, and one space after the mark. Note how Kevin and I have done this. Apart from that, the important thing is that each sentence should represent a clear thought. If your sentence is turning into a long line of words, perhaps that's a sign that it should be two or more smaller sentences.