My knees are cracking! Help?!

Yes its normal. Its just nitrogen bubbles popping. Its the same thing as when you crack your knuckles. The gas that's causing it is what causes the bends in deep sea divers. You will be fine, so don't worry.

Extremely normal. Cracking just relieves excess stress, especially if you do many sports. You'll notice many ballet teachers crack their knees when they bend down, it happens with very flexible people.

It's normal, especially if you're going through a growth-spurt. If it starts to hurt though, tell someone!

It's nothing. Your knees are just... stiff. I dunno. Something like that.
Don't be so paranoid. It's not a big deal.