StephenH

Assuming the bike is not obviously bent or has loose handlebars or something weird like that, it sounds like it's just a matter of getting used to riding again.

One thing that makes learning to ride harder is that it is easier to ride straight when you're going fast than it is when you're going slow. And of course, if you're not too confident, you're going slow. But just keep at it and you should see some improvement. If you can find a big parking lot without much on it or get on a wide bike trail at a non-busy time, just ride around for practice. Generally, don't pay attention to how straight you're going so much as just watching out in front of you where you're going, and the going straight sort of naturally happens.

I'm not sure about the starting and stopping, but I would say that generally, if what you're doing works, don't worry about it a whole lot.

Generally, yes, you'd be in first gear for hills. More specifically, that's the one where the pedals spin fast and the bike goes slow. Usually, when I hit a hill, I just start up in whatever gear I'm in and then downshift as it gets harder.

A common issue for new riders is seat height. If you have your seat down too low, it makes it harder on your legs when pedaling. Ideally, your leg will be almost straightened out on the downstroke. If you want to look cool or jump your bike over stuff, you keep the seat low, otherwise, raise it up. That will help some on the hills.

Something that will help on a "mild incline" is hitting it faster in the first place.

When I first started riding again, I noticed some fairly fast improvement in toning my legs up, so you're likely to be going quite a bit better in two or three weeks than you are now.

If all you do is ride your bike a half mile or mile two or three times a week, you may not be riding enough to see much improvement in it. Get out and ride an hour every evening, and you'll see more effect.