push to talk

The default key for push to talk is LEFT ALT.

Check in your settings for keybinds to make sure what key it is set to.
Also, turn on to show your talking icon (I think this is in gameplay settings, but I'm not positive right now - when I check, I'll edit this). Having this on will tell you whether you are activating the push to talk or not (you'll see a little speech bubble at the top right corner of your screen).

If all of that is showing right, and you're still not coming through in the game - the problem may be in your Windows settings.

In Windows, check Settings - Sound.
Check what is set as your default input device. Make sure it is the mic that you want to use.
Also scroll down to Other Sound Options.
Click on "App volume and device preferences".
With Sea of Thieves started, the game will show up in these settings, and you can check to see if the correct mic is set to be used for the game.

Hopefully that all made sense AND helps you to get it all working for you!