This one depends on what wheel and truck you have, but check it out. There is also a entire kit you can get from this website. diy-electric-skateboard-kits-parts/single-bolt-on-motor-mount-only/
First, what’s your desire speed? Second, the range that you want. Third, how much you willing to spend?
You got the most essential parts to build and electric skate but you need the skate before you can build the thing.
You will need:
deck
grip tape
2x trucks
4 wheels
8x bearings
motor mount
pulleys
belt
battery charger
male to male connect for your vesc/esc
…
Long story short, watch this video. You’ll get an idea of what to buy.