I have done zero prep before. Basically its the classic "you all are in a bar and X happens" and then we go.
A few important notes:
1) Now the key is, you have to be exceptionally good at improv as a DM. If you aren't relying on plans, then you have to be able to pivot and create on the fly.
2) You need a group that self motivate. People who have character motivations they are not afraid to make the stars of the show. Aka they see a hook they go for it, or even better, generate their own hooks based on their characters. A group that tends to want the DM to lead them around doesn't work as well in zero prep.
3) You have to take good notes (or even better, have a player that takes good notes) if this is meant to be more than a one shot or two. While you don't need to prep ahead of time, you do need to start recording what has happened, as well as all the names, places, etc etc you make up on the fly. If you don't you will quickly lose track and the game will lose cohesion.
4) Honestly the encounter part is actually the easiest part imo. You can use stock CR encounters for your group until you get a feel for how strong they are, and 5e is so forgiving imo if I throw something a bit too challenging the party usually can work through it. I would go with theater of the mind so you don't have to build battle maps and the like.