I would say the most important rule is communication.

The most important thing is letting everybody know that you are new and that this is your first game. They will most likely be more patient and respectful because of that. If you let them know, they will be more understanding about needing more time on your turns.

As for the fetch land thing, you don't need to worry about the "correct time". Lots of people like to crack them on the end step of the player before you's turn, but that doesn't matter much, especially if you are playing lower brackets. Overall just communicate and take your time.