Making the switch from an adult TRI-cycle to a hybrid BI-cycle took some doing. I decided to have the pedals removed from the hybrid temporarily, and the saddle lowered so that I could sit on it with my feet flat on the ground. That let me push myself along, like riding a Draisine. After about two weeks of practicing glide biking, I had the pedals replaced. I didn't really feel ready; but something told me that, if I waited I felt "ready", I might never do it! I also had the saddle raised a little. Not to full height, though; I could still reach the ground on tip-toe while seated.
There are things I wish I'd had somebody to tell me when I was working to adjust to two wheels. For starters -- well -- how to start! It took a long time for me to figure it out. Sure, you press down on one pedal while using the other foot to give a little push off the ground, but there's more to it than that.
So, if you're just starting out on two wheels, especially as an adult, I hope this can help:
In those early days right after having the pedals replaced, I would have done much better to not even try getting onto the saddle. I'd try to get there so fast that I often snagged the back of my shorts on the nose of the saddle. Then I'd have a hard time not losing control before getting un-snagged and stopping before having another try.
A better method would have been to get the starting pedal into the "two o'clock" position (this is done standing over the bike, of course); push it down to start myself rolling; glide a short distance while leaving the other foot off its pedal; and then, as I lost momentum, brake and put the non-working foot down.
After I got to feeling in control doing that, I could have started practicing getting myself onto the saddle, but still not trying to use both feet to pedal. Sit on the saddle; then rise up on the working leg (but not letting the knee lock); and brake; step forward and down. When that got to feeling natural would have been the time to start working at getting onto the saddle and putting other foot onto its pedal, and trying to pedal using both feet.
At first I didn't realize that that I wasn't letting myself rise up high enough when I started. No wonder I kept snagging the seat of my shorts on the nose of the saddle! The knee of the starting leg needs to become almost straight (just not so much that the knee locks). I also need to lean forward slightly at push-off. Another thing that helped was, I finally realized that I needed to pull on the handlebars in a slight towards-me-and-upwards motion. It helped me to rise up high enough. Then, once I'm rolling, I can get onto the saddle.
When I had the saddle raised to full height, I quickly learned that I couldn't put a foot down to the side when stopping, which was my automatic impulse; and what I had done when the saddle was still low. Trying to put a foot down to the side when the saddle was at full height meant that the bike's weight would almost push me down. This was the point when I realized that I needed to rise up on one foot (as in starting, leg almost straight but don't let the knee lock) and get the other foot ready to step down; brake; and step FORWARD and down. How far forward? Enough to clear the saddle's nose, so you don't snag your clothes and pull yourself over.
Those are the basics as well as I am able to cover them. Every rider is different, so every rider needs to experiment with these things and find out what works best. I need to get this post out, because it's getting harder now for me to remember how difficult riding a two-wheeler once seemed. If I had ridden continuously since childhood, I probably wouldn't remember at all! Learning to ride a bicycle can be rather discouraging at first. They say your body learns how to handle the bike, and it seems to be true; but the learning curve can seem both steep and endless. But don't give up! Take your time, and keep at it. It's worth it.