Punctuation exists for one reason: to make your meaning clear to the reader. We're going to keep this simple and focus on the three you'll use in every single scene you write.
A period ends a sentence. Full stop (pun intended). When a complete thought is finished, put a period at the end and start a new one.
This is a complete sentence.
The comma is the most misused punctuation mark in fiction, mostly because writers either sprinkle them everywhere or avoid them entirely. A comma signals a brief pause, and it has a few reliable jobs:
It separates items in a list: She grabbed her keys, her coat, and her phone.
When in doubt, read your sentence aloud. If you naturally pause, a comma probably belongs there. If you don't pause, it probably doesn't.
Quotation marks wrap around the spoken words of your characters. In American English, you use double quotation marks ("like this"), not single ones.
The punctuation at the end of spoken dialog goes inside the closing quotation mark, not outside — always.
"I don't think that's a good idea," she said.
He shook his head. "We don't have a choice."
That's it. Master these three and you'll have a solid foundation for everything else.