My Mac computer is starting up with a folder with a question mark in it what do I do ?

No, it isn't true. You should be able to reinstall the OS by booting into the Recovery Partition unless you somehow erased the entire hard drive. Boot to the Recovery Volume (command - R on a restart or hold down the option/alt key during a restart and select Recovery Volume). Run Disk Utility and select First Aid. Then re-install the OS.

Reinstall from macOS Recovery

OS X Recovery

"If your Mac starts up to a question mark", click here --> https://support.apple.com/HT204323

Unless you are using a truly ancient Mac then booting to Recovery should take care of it. You'd also have to be using a truly ancient operating system version to be using one from the days when you had to pay for it.