My left knee has been giving out when i have been jumping in the trampoline the past day or so and cracking a bit. is this normal for a teenager?

Not normal: "Cracking" could be normal but your knee giving out is not. Most worrisome would be that you could have injured your neck, if you forced your head position during a jump or fall.

Cracking without: pain can be normal, but giving out while jumping or cracking with pain is not. You should see your primary care doctor and/or an orthopedist to get examined to make sure there is not a structural reason for this, such as a tear in one of the knee ligaments.