If you ever rode a bike before you will never completely forget it. It's in you for life. Have someone look at the bike and make sure it's working properly and set up correctly for you. Dress comfortably, go to an empty parking lot and practice.
Wear a helmet. A minor, sudden fall can be unpleasant. I'm not getting into another helmet discussion here. It won't save your life when a truck runs you over but in case of a minor spill it can make a difference between a small bump on your head versus a trip to ER to get five stitches.
Don't use clipless pedals or toe straps, just flat pedals and comfy sneakers.
Put the bike in an easy gear, shifting will come later.
Look forward, sideways but never, ever look down on your wheels or pedals. Don't look down even when you're stopped and getting your foot on a pedal: let your foot find the pedal without the help from your eyes.
Don't THINK about your balance, don't think about falling, think about going forward. If you start thinking about your balance you will lose it. Balance on a bike comes natural once a person overcomes the fear of falling.
Before you can learn to ride, you have to learn to stop. Practice safe stopping until you're confident that you can stop your bike. When stopping DO NOT take your feet off the pedals until the bike almost completely stopped moving. Then you take ONE foot off the pedal, lean to that side and put that foot down. The other foot STAYS on the pedal! You'll need to figure which side is your favorite.
Put the saddle low, so you can reach the ground while seated. Relax, loosen up, sit firmly on the saddle, arms relaxed, palms relaxed, easy grip, don't look down. Put one foot on the pedal, don't look down, push with that foot while you let your other foot find the pedal, don't look down - rotate a couple of times, break GENTLY using both brakes and stop. Repeat until you can stop without much thinking. Never, ever look down!!!
The "handlebar grip" thing is important almost as much as the "do not look down" thing
if you death-grip the grips and stiffen your arms you're more likely to jerk your handlebar and lose balance. Keeping your front wheel steady is the key to your balance, any sudden moves will upset it. You do that by keeping your arms bent and palms relaxed. If a bike is set up properly it should have a natural tendency to go forward more or less, so you need to remain relaxed.
And yes: congratulations. You're doing a great thing. Take it easy, take it slow, don't get discouraged. If in doubt, post here for moral support!
Oh, if the bike is indeed too large, try to sell it or trade it for something the right size or you may have really hard time, no fun, and pick up bad habits.