1. Use a full stop at the end of a sentence:
2. Use full stops with abbreviations (in an abbreviation the last letter of the word and of the abbreviation are not the same):
3. Do not use full stops with contractions (in a contraction the last letter of the word and of the contraction are the same):
The rule about abbreviations and contractions is not followed by everyone. Sometimes it is a question of style. The important thing is to be consistent.