So here's my best understanding of the mechanics of the first turn of combat:

If you use an action to attack someone and it starts combat, everyone rolls initiative and takes turns in order. When your turn comes around, you'll be missing that action--basically, you to use your action before everyone else got a turn, but that counts as your action for the round.

If you engage turn-based mode out of combat and your actions initiate combat, it works the same way. I this case, you might also have used some movement and/or your bonus action.

There are some abilities that grant all your actions, bonus actions, and movement back when you roll initiative (the Assassin gets this, I believe). So if you spec'd Astarion as an assassin, he'll "break" the above rule and have all his resources available, even if he did a massive sneak attack to start combat.

Even in real time, if you do something like jump, you'll notice your bonus action icon goes gray for a few seconds. If combat starts in that window, you'll have already used that bonus action for that round.

Hope that helps clarify!