The first thing I'd recommend doing is lowering the seat until it's at a level that while sitting on it your feet can both touch the ground. This will mean that if the bike is ever about to fall you'll be able to put your feet on the ground comfortably.
Sit on the bike seat and just use your feet on the ground to push the bike forward and get some motion. See if you can get going fast enough that you can temporarily lift both feet from the ground and sort of just glide on the bike. Do this until you get the feel for balancing. Forward motion is really critical at first so don't try to force the bike to go really slow. The bike going forward means it's less likely to just fall over.
Once you can glide alright try pedaling a little bit. If the seat is still down low you may want to raise it up so that pedaling is more comfortable. Again, forward motion is critical. If you're moving forward and the bike starts to fall sideways it'll just turn. If you're not moving forward and the bike starts to fall you'll just fall.
At slow speeds it's alright to turn the front wheel in order to steer. At higher speeds though you're just going to be leaning in order to turn. With a little practice you'll get the hang of it and it'll basically be second nature.
