Combat is turn-based and starts with initiative. On your turn, you usually have:
Movement (you can move in a single turn a distance equal to your speed. You can do this before or after your action (you can even split it to before and after))
One action (this is when you can do most things, most importantly, attaking)
One possible bonus action (bonus actions happen at the end of your turn. It’s smaller than an action)
A reaction (Used outside your turn. Most commonly used in opportunity attacks which you can do whenever a creature moves out of your range)
Free interactions (Like drawing a weapon. Depending on your table, some things might be free actions while others aren’t. A common house rule is that drinking a potion is a free action, even though officially it’s an action.)
This structure keeps combat organized while still allowing tactical freedom.