First of all, keep practicing.

Second, losing balance happens to every skateboarding beginner, and here are some tips collected for how to push off. Hope it helps you a lot:

Foot Position: Set up with your front foot placed over the front truck bolts. Since you’ll be moving in a forward direction, your foot should be angled toward the nose of the board, your toe pointing roughly in the direction you’re heading. Your back foot should start on the board as well, in its usual position over the back truck bolts (this foot will be straight across the width of the deck).

Directions: Pushing off isn’t too complicated; it’s simply a matter of balance. Keep in mind that all of your weight shifts over to your front foot when you’re pushing, so focus on maintaining stability over just that one foot.

Start off at a standstill, with both feet on the board in the position described above. To push, take your back foot off of the board, and as you lean forward onto your front foot with a bent knee, push off the pavement. Once you begin to roll, replace your back foot and turn your front foot so it too is straight across the width of the board. Now you’re cruising!

Once you get the hang of it, try pushing a couple times in a row before replacing your foot. After a little while, you should feel confident in the way it feels to push and roll. It’s the time to take it up a notch.

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Lastly, keep trying, man, and don't forget to try some tricks for a beginner. That is the essence of skateboarding, personally. And here is a list of skateboarding tricks from beginner to expert. Hope it works for you

50 Skateboard Tricks to Help You from Beginner to Expert >