Watch the Tremolo Exercise: Slow online guitar lesson by Marcy Marxer from Mandolin for Guitar Players
Tremolo is a fast up and down motion on one or more strings. Practice slowly at first keeping your tremolo very even. Count 1, 2, 3, 4, 5, 6 for Down, Up, Down, Up, Down, Up. Set metronome to about 72 BPM. Each metronome beat will be played as a triplet. Count the click as 1 and count 2 and 3.