It's not hard to ride a bike, but learning to balance correctly takes a bit of practice. First you need a bike to learn though.
BIKE:
I know, most department stores sell bikes . . . but these bikes suck balls. They're heavy, poorly made, use crappy components, and typically aren't worth your money. They will suck to ride long term. The cheapest way to get a bike is to look for a decent brand used one (Giant, Trek, Fuji, Specialized, Cannondale, Scott, etc.), however as a completely new person to the activity it will be difficult for you to figure out what to look for. So it's probably best to go to a bike store and get a real bike (type of bike doesn't really matter, you just want the bike to fit you*). Try to stay away from anything fancy - you don't need a front or rear suspension, you don't need fancy brakes or electronic gear shifting. You're probably safe to aim for something like a non-suspension mountain bike or a hybrid bike if you're more comfortable with flat bars, maybe a touring/cyclocross bike if you're interested in drop bars. Buying from a bike store will mean that the bike will be properly set up and in good working order while you're trying to learn to ride it, which is important.
RIDING:
As far as actually riding the bike, I taught my 5 year old son to ride a bike the same way I taught my 35 year old wife. Lower the saddle farther down than you would want for regular bike riding - you want to be able to get your feet flat on the ground on either side of the bike while learning. You want to find a big, mostly flat area to play around in (empty parking lots are good for this, as are quiet residential courts). This is a graduated process. Don't move on to the next step until you're comfortable with the previous one:
- practice scooting around with your butt in the saddle, pushing with your feet
- scoot faster until you can lift both feet off the ground for a couple seconds
- do the same scooting, but learn to use your hand brakes to slow yourself down
- now try scooting even faster, and see if you can coast/balance for slightly longer periods
- find a slight downhill/ramped area (not too steep) that you can roll down. (Ideally you want a long flat area after the ramp.) Then scoot down this and balance for longer periods of time (20-30 seconds is ideal).
- Then practice going down the ramped area, and putting your feet on the pedals, gently slowing down with the brake, then putting your feet down on the ground.
- After that, you want to practice leaning the bike gently to steer it left and right.
- Next up is putting your feet on the pedals and actually pedaling the bike.
Then it's a matter of practicing regularly (ideally you want to do this every day for 10 - 20 minutes) until you're feeling like you're in control of the bike. Ride over different terrain (sidewalks are bumpier than parking lots and require different handling, riding on grass is very different than on pavement, etc.) and practice handling the bike. Learn to shift gears and what the different rings/cogs do.
As you do this, you're going to want to raise the saddle height higher. (Being able to get both feet flat on the ground on either side of the bike is much too low for proper pedaling mechanics and will make cycling much more physically tiring than it should be.) Raising the saddle higher will require a different starting technique for riding the bike (good info about the best methods of starting/stopping here -
https://www.sheldonbrown.com/starting.html)
GEAR:
- I'd recommend that anyone who buys a bike also buy a track pump with a pressure gauge and keep it in your garage next to the bicycle. Bike tires go flat over time (air actually leaks out of the butyl inner tubes) and you'll need to do this periodically for as long as you own the bike.
- You need a bike helmet, but you don't need anything fancy. The cheapest bike helmets protect just as well as the most expensive ones. Just find something that doesn't look too ugly, and make sure that you know which end is the front (don't laugh, I had a co-worker who was wearing her bike helmet backwards).
*A bike store should be able to guide you towards a bike that fits. I could write pages of info about this, but basically if you're new - listen to what the guys at the bike store say.