Originally Posted by Skipjacks

Green slime is a pain in the rump. It can (will) clog up your valves. If the tear in the tube is bad enough the slime will just get all over the wheel requiring it to be cleaned.

For me the best option is to carry a spare tube. They fold up really small and fit in most frame bags and pouches or just zip tied to the frame. That said in 15 years of regular riding I've never blown a tube. (Why do I say these things? Must I mock the biking gods? This will bite me I'm sure.) Anyway that's with 26" wheels with 2 inch wide tires and 29" wheels with 38mm tires. Blown tubes are more common with thinner road bike tires because they don't have as much volume to flex against. And unless you are riding through literally fields of thumbtacks and nails the odds of a puncture going through the tire and putting a hole in the tube are slim. Tires might be punctured but the tire is just tread. You can put 100 holes in it as long as the tube is good underneath. (I'm assuming you're not planning on running a tubeless set up)

I'm not saying not to carry a spare tube and a pump because I always do. I've needed the pump more than a few times when I just realized tire pressure was low well after I'd left the house. But the green slime...you can live without.

Other tips...

Pannier bags are your friends. They are special bags that attached to the rear rack on a bike (also get a rear rack) and hang to the sides of the wheel. They let you carry whatever you need to with a low center of balance and without having to weight you back down with a back pack. Some people love backpacks, I hate them. So this is a personal preference thing.

A front and rear flashing light is REALLY important if you'll be riding on roads with cars. Look at Cygolites for examples. They flash REALLY bright white front and red rear to make sure you are seen by drivers. They work really well.

And a helmet. Don't cheap out on a helmet and get a $13 piece of trash on Amazon. Or any helmet on Amazon because it's likely to be a knock off that hasn't met safety standards. Go to a store and try different helmets on to make sure you get one that fits. Spend more than $20.

As for bikes, go to a local bike shop (NOT WAL MART) and see what you like. Try different sizes. Different models. Different brands. A Specialized and Trek similar type bike with similar sizes might ride very differently. And one might be perfect for you but horrible for me. So you really have to get on various models and see which one fits you the best.

Also, what's your budget?

Also...whatever your budget is, set aside double that for all the fun accessories and other stuff you decide you can't live without over the next year after you buy a new bike. Or call your credit card company and tell them to block charges from the local bike shop if you don't want to spend future money. It gets addictive.