Get an idea of the range, speed (and recovery) and height of all your normal moves and when to use them (it’s ok if you focus on the ones that seem better, but try to recall that the others exist too, and incorporate them when you feel you need more variety).
Make sure you know how to block. Not as in becoming a blocking master, but a general understanding of when to block high or low, when people are likely to try a sweep or an overhead…
When people jump at you, try to think if you have time to antiair them. If not just block. You’ll eventually see them coming and you’ll be able to antiair properly. It’s easier if your character has one really good antiair move that you can keep in your head ready for the situation. Also be aware that opponents can (and may abuse) jump just over you, and their air attack has to be blocked in the opposite direction.
Put focus on throws and throw teching when you feel you need it, this is, when you see people always blocking your moves or you see people throwing you much.
Special moves, they are important, especially depending on the character, so I can’t say don’t pay attention to them from the start. At any rate, understand what they are useful for and what are the risks of doing each.