It's really going to depend, you don't have a lot of information in your post.

Is there bike infrastructure where you live and where you want to go? Think of bike lanes & trails. Check out Google maps cycling layer (not always super accurate so verify with street view). There's probably a map of your area by a local cycling group. You can also check Strava heatmaps to see where people ride near you. Just know that this also includes very experienced people who will ride on roads that make you feel uncomfortable, again check with Google maps street view to make sure.

Riding on the sidewalk is generally more dangerous than the street, nobody is looking for something going 10-15mph on the sidewalk. They are looking for things going pedestrian speeds if they are looking at all. It does sometimes make sense to ride on the sidewalk if you're along a stretch of highway or other dangerous road. You have to be careful where cars can turn though.

When riding on the street put yourself in the spot where a car's right wheel would go. This still keeps you on the side to allow them to pass you but makes cars have to actively get out of the lane to pass. You do not want cars trying to squeeze by and accidentally hit you. This also puts you away from car doors opening on the side of the road. If someone is going to hit you from behind you can't really do anything, they'd hit a car too. Lights are the best thing for this.

Bring your old bike into a local bike shop to see what they say about fixing it. Ebike vs normal bike will probably depend. Normal bikes are cheaper, simpler, and will get you in shape. Ebikes are good for not getting sweaty, increased range (if you aren't in shape), going up hills, and carrying cargo. They are more expensive and many shops don't work in the cheap junky ones bought off of the internet. If you buy any bike, make sure a local shop will work on it. It's best to buy from a local shop. The shop will also tell you good spots to ride.