My knee gave out and now it's in constant pain, is stiff, and keeps giving out. What did I do to it? I was playing basketball and I must have landed on it wrong because I got a sudden pain and my knee gave out and I fell. Now it is super stiff and it hurt

Get checked out. You should see a sports medicine orthopedic surgeon, to check if you've torn your acl. If it is ok, then there could be other findings responsible for your knee giving out.
This . This sounds like a possible injury to a ligament in your knee. A diagnosis can be made by your primary care provider, or an orthopedist. An x-ray followed by an MRI will likely be ordered. This is a situation where you should be evaluated instead of trying to "fix" the problem on your own. You may need surgery depending on the type of imjury you sustained. In the meantime ice your knee, rest it, and use an ace bandage (not too tight); you can also take an ani-inflammatory medication for the pain and swelling (i.e. Advil (ibuprofen) or motrin). Good luck to you.