Get a metronome. Or if you are on a higher budget, get a drum machine.

Learning how to subdivide is also very useful...

Example:

If you are on Common time (4/4) and you play Crotchet(1/4 notes) then you count each of them as, 1 2 3 4


If you are on Common time (4/4) and you play quavers(1/8 notes) then you count, 1 n 2 n 3 n 4 n


If you are on Common time (4/4) and playing semi-quavers(1/16 notes) then you count 1 e n a 2 e n a 3 e n a 4 e n a



You may find subdivision very useful when you play songs that have many "Off-beats"


Example: If you are on Common time again (4/4) and you have to play

Dotted Crotchet(1/4+1/8) followed by a quaver(1/8) and then again Dotted Crotchet(1/4+1/8) and a quaver(1/8)


Subdivide in "Quavers" and Count out 1n 2(Dotted Crotchet) n(Quaver) 3n 4(Dotted Crotchet) n(Quaver) and so on..

Jazz is all about knowing how to "Swing" literally and musically