Brought this bike for $100 , need new tyres/tubes as they are flat

Your tyres will need topping up once a week. Are they staying inflated for 24 hours?

First off—congrats on the bike. I’ve had thousands of great miles on a $100 bike and I wish you the same.

These wheels have inner tubes that hold air and tires that protect them/grip the road. They might just need to be inflated with a bicycle pump—something you should do every few days to a week. Or, they might be old and need to be replaced. Tubes are super cheap, and changing one is an important skill to learn because flat tires happen often. It’s also good practice to keep a tube and a tool to change them with you.

The tires are more expensive, if they need to be replaced—maybe $50 each for a decent tire.

It sounds like you have some basic skills to learn, so spend some time on YouTube or the local library, and go ask your local bike shop for some advice. If you can afford it, get them to do a regular service on the bike. But when you pay $100 for the bike, everything else is going to seem expensive.

Good luck!