OOC
The way I understand it, the action economy works by giving you 3 actions and 1 reaction a turn to spend. Some actions like certain spells or attacks cost 2-3 actions to perform. You can make 3 basic attacks if you so choose, but there is a multiattack penalty if you do (-5 second attack, -10 third attack), making second and third attacks usually useless without outside help.
Reactions usually just use your reaction slot or sometimes need the prepare action to activate. for example, Aid needs you to prepare first to aid a fellow player's specific skill/attack roll, using the Ready action, which takes 2 slots.
On the other hand, Attack of Opportunity (a fighter only feature now) can be used whenever it is triggered (a creature leaves your range, etc). I will just not keep track of your class or specially gotten reactions though so it's your job during combat to mention stuff like Attack of Opportunity so I can roll for you when your enemies trigger it.
For now, I'll leave these aspects out of the equation until full combat starts in game (not quite there yet). I'll give you a roll to try to interrupt his action though.