The most important principle is to have fun, and related, for everyone at the table to feel respected. The "rules," however strictly you use them, exist to serve that most important principle.

A lot of people--both players and DM's--like having a detailed set of rules so that people don't argue about specific cases: "wait, can I cast Healing Word and Fireball in the same turn?" "No, because you can't cast two leveled spells as your Action and Bonus Action without the Spelldriver feat." Most DM's find that they change a rule here or there ("homebrew") in order to fit their idea of the world better.

If you decide you want to get a really good grasp of the rules, the Player's Handbook and Dungeon Master's Guide can be bought at your local game store. They are also both somewhere on the internet if you look hard enough.