1991 Honda Civic DX Hatchback with key/lock issues

Bought this car last week. So far having problems but keeping hope.
One key to the car but it doesn't work for any of the doors, the trunk, and works in the ignition with some jiggling and pressing the brake usually helps.
Is it a bad key?
Jammed locks from rust/water?
It fits into the keyhole but not turn, honda owners where can I start?

All I can tell you right now is: The only key you have was "Keyed" to the ignition cylinder. The ignition lock cylinder was replace before and whoever did it not making (Re-Keyed) the new key to fit the doors lock cylinders or the trunk lock cylinder and the tumblers are worn out.

You've already purchased a 24 yrs old vehicle so the only right thing to do is buying all of the lock cylinders for the doors including the trunk and the most important: The ignition lock cylinder and its ignition switch, the most notoriously failed component when it come to stalling or fail to start in Honda vehicle in the same era.

I'm sorry for the solution as most of the lock cylinders are the near of its service life. It would be the best if your intention to keep the vehicle in a safe and convenient way to start with.

Edit: If you are mechanically inclined and want to taking care and repairing the car. E mail me and we'll talk. Who knows? It may save you some headaches and cocos. HTH.