Learn the difference between your and you're. Mixing up these words is one of the most common and easily-fixable usage errors in English. Since there is nothing that stands out more quickly than writing "You're not coming to the dance, are you?" it's important to clear up the difference and never make this mistake again.
Your is used to mean "something you own." Saying "Is that your cantaloupe?" or "Where is your pocketknife?" are appropriate uses of "your." You can always sound it out and try substituting "you are" for "your" in a sentence. If "you are" would make sense, then you should use the other form "you're."
You're is a contraction of the words "you" and "are," and it's used as a replacement for those words in a sentence. "You are an excellent tennis player" can be written "You're an excellent tennis player.