Physical Therapist
I’ll get the preachy bit out of the way first:
Find a local bicycle shop, have them sell you a helmet, water bottle and cage, and show you how your gears work. See if the cycle shop runs maintenance classes for beginners, then buy a pump and patch kit as a minimum. If you can’t fix a flat never ride farther than you’re prepared to walk.
Learn to ride in traffic. In many places riding on the sidewalk is illegal unless it’s a designated cycle path or shared use path. Check your local ordinances.
Don’t worry too much about the weight of the bike holding you back, unless you’re riding with a group on racing bikes. I’ve done 100 mile rides on a mountain bike with slick tires with no real problems.
See if there’s a cycling club in your area. They usually have regular rides for various abilities and will know where the good rides are.
When you’re just cycling for fun a level of effort where you can hold a conversation is about right. As you get fitter you’ll find your speed at that level of effort will increase.
If you have a smart phone there are loads of apps available to track your route and speeds.
FWIW, I know three people who are probably alive thanks to helmets.
One was crossing an intersection where he didn’t have a stop sign, and a car blew through the opposite sign-- or started to, and he went tumbling over its hood, and splat on the opposite pavement.
Another hit a pothole, and flipped, and landed on the top of her head. She had a recovery that lasted several months, and was kind of shaky for a little while, and her helmet cracked like an egg. Without it, that would have been her skull.
Third was a kid who hit a fence going downhill really fast. He had a bloody nose and a fat lip, and had to have the front wheel on his bike replaced. He also had a crack in the outer plastic on his helmet.
Now, the second one is the only one who I can say absolutely would be dead without a helmet, but the other two might have spent a long time in recovery, and acquired permanent disabilities.
I know one person who told her kid that a helmet is a lot more cool than a catheter, breathing tube, mouth controlled wheelchair, or walking with crutches. Also, easier to fit, I might add, than an artificial leg.
[/preachy]
Gears
For efficiency and to prevent strain injuries to your knees you should aim to have a “cadence” of around 80 - 100 rpm. That is, one foot is making a complete circle between 80 and 100 times a minute. You can get cycling computers that measure your cadence and display it to you but you can get a rough idea easily enough by counting your pedal strokes for 30 seconds (and doubling the result obviously). With a bit of familiarity you will know by feel when you have a good cadence.
In order to get an optimum cadence you need to select the correct gear. A hard gear will have you pedalling too slowly or will require too much effort to pedal at a proper cadence. A gear that is too easy will have you pedalling too quickly or not putting enough pressure on each stroke and you will be travelling slower than you could be.
It will only take you a couple of rides to get familiar with your gears. Experiment with them and see how they work and how they affect your exertion and cadence. When you buy your helmet take your bike into the shop and get them to give it a quick check over to make sure gears and brakes are working correctly.
Once you are familiar with how the gears work remember that it is normal to change the gearing quite frequently so that your cadence stay moderately high (80 - 100 rpm). When you go up hill you will need to progressively select easier gears to keep your cadence high. If the hill is steep enough you will “run out of gears” and then you just need to accept a low cadence. The opposite applies when going down hill.
You may also choose to use harder gearing and a low cadence for short periods of time in order to increase your leg strength but it sounds like you are not at the stage where you are using your bike as a specific training tool so I’d stick to the general rule of keeping your cadence high unless an uphill section forces you to use a lower cadence.
Helmets
To add to the preaching: I don’t have a choice not to wear a helmet as it is a legal requirement in Australia, however I can say that my (old) helmet has certainly saved me from suffering more serious injuries than I did in an accident. I fell off during a race and my head hit the ground hard enough that the helmet was broken completely through in several sections. I suffered no head injuries other than a minor graze though (unfortunately I also broke a collar bone and ripped a finger nail off, but those things heal much better than brains do!)