How does one input donation write offs during tax season?

I’ve never known the complete details of how a write off actually works. I’ve donated to the thrift shop a couple times, but never really knew what to do with the slip they gave me.

Also, when doing taxes, do online donations count? Can I insert that information too? For instance, if I fund a KickStarter project can I receive a tax write off?

Thanks so much jellies!