I learnt to ride a bike when I was about 8 years old. I can remember that the bike I tried to ride first off was too big for me, and I couldn't even sit on the seat and reach the pedals properly, which made it hard to manage. Back then, the bike didn't have gears and I pedalled backwards to brake. (It was way back in the '60's)

I think that you need to lower the seat so that you can touch the ground with both feet while sitting on the seat. Next learn how the brakes work and use them both at the same time - not just the front one. Forget about changing gears until you become more proficient at balancing and maintaining control of your steering. Just have the gears set so you can pedal comfortably, but not too easily (ie spinning furiously). Keep practising and then gradually learn to change gears, and then begin raising the seat as your ability and confidence increase.

Congratulations for taking the plunge, you won't regret it!