Here are some of the most common errors I see:
Their (possessive), They're (contraction for "they are") and There (in reference to a location).
correct example: They're so happy they found their toys over there by the tree.
Your (possessive) and You're (contraction of "you are")
correct example: You're as good as your word.
Too (meaning: also), Two (numeric), to (most commonly used to express motion toward something)
correct example: The ants marched two by two to their ant hill and the queen went too.
Whose (possessive) and Who's (contraction for "who is")
correct example: Whose coat is this? Who's going to the party?
I know there are many more out there but the point is to take a moment and check your spelling to be sure you're using the word.