for z cancel and short hop just go into training mode, vs any character it doesn't really matter. Do 100 short jumps in a row. If you mess up, start counting over again. This will make it so you can short hop on command at any time. For Z cancel, pick link and do a short hop (which you now have perfected) and then your dair, and z cancel it 200 times in a row. If you mess up start over again. Then play your main (falcon) and just practice doing his short hop air attacks, and Z cancel all of them. Do 100 of each (nair, fair, bair, dair, uair) until you can get it perfect every single time.
This method is pretty boring, but if you really want to improve at these skills, here's a routine.
For teching, just think of it like z cancel, and hit z when you're about to hit the ground.
And one last thing, stop saying you suck all the time. you aren't really that bad, I've seen way worse. If the whole time you're playing you're just thinking about how much you think you suck and how terrible you are, well then you aren't going to do good.