If you should find yourself in the quandary of "Do I use an apostrophe or not?" then ask yourself: "Am I trying to say "it is"?
It's really hot outside!
Should've/Could've/Would've - these are short for "should have/could have/would have". You should never ever say "should of", "could of", or "would of". Those don't make sense because they're incorrect. What you should be saying is:
I should've asked the grammar marm.
I could've washed the car, but I watched some great Youtube videos instead.
I would've removed the spinach from my teeth if you would've told me about it!
You're - this is short for "You are". This is very important. You absolutely must understand the difference between "you are" and "your". Totally different.
You're the smartest man I know!
Is this your pocket protector?
I hope this radically changes your life as I know it will.