I don't know what a Nair is and tilting that's confusing and how to do them. Asides from knowing terms and the techniques and stuff like reducing fall time and time on the ground. Is knowing you're character and getting good with it the most important thing?
N-air is neutral air, just as b-air is back air or f-air is forward air. D-air is down air and U-air is up air. Basically the letter is just to specify which aerial it is. N-air is just pressing the attack button in the air without holding any direction. That would be the one where Fox extends his leg horizontally.
Tilting is holding the stick slightly up, down, or to the side and pressing the default attack button. You can also do them while walking; however, you cannot do them while dashing or you will get a dash attack. Up-tilts are tricky if you have tap to jump turned on. You can turn it off in the controller settings and I would recommend it myself.
Yes, knowing your character inside and out is probably the most important thing about the game aside from proper spacing and reading your opponent. Understanding what Fox's kill moves are and at what percents they will work on what characters is imperative. Realizing which moves are safe and which are not will make you less punishable. Utilizing his tools correctly is also important, for instance, you can use lasers to force an approach or reflector to stall yourself in the air thus mixing-up your landing.
