Metronome
I’ve been told by countless music teachers that the most important quality to have as a musician is solid rhythm. The only way to know for sure if your rhythm is solid is to use a metronome or play with others. The next most important tip is remember to listen to the metronome or the others with whom you are playing. Do you ever find yourself using the metronome and then completely ignoring it? If you do, take a break and come back to it.