Tremolo is a very important technique all kinds of guitar, especially today.
However, it is very difficult to learn; you will have to work on it very regularly.
Now, let's start with the theoretic part.
Tremolo is the picking of one string, generally the 1st or 2nd, three or maybe more times, but so quickly that it has to sound like one sound, like a wave.
Practice with a sturdy pick since you will have to shred.
Before, just revise down-up-down-up picking style.
You must make the littlest of hand movements.
You will have to use a metronome.
You can try with 80 bpm for example.
When you hear the tone, just play the 6th string then the 1st string 3 times.
If it is too hard for you, then just try 5 bpm slower.
When you are comfortable, practice like for 10 minutes non stop, then just try 5 bpm quicker.
Until you reach a good speed.
It is very important for you to play the note good, there should not be any undesirable sound.
Make sure you play it accurately and regularly, every note should be heard.
You have to play every pattern at the same speed.
Here is an exercise you can work on with your metronome: 1st string 14 0 14 0 12 14 0 14 0 12 14 12 0 12 0 12 0 etc.
Repeat this pattern again and again...
Keep practicing and increasing the bpm (beats per minute) of your metronome until you reach a good speed.