You're having a problem to open a locked door. But you're sure that you have the key to access into the place.
First, make sure that you have the key/access into that restricted place.
And of course, the door that's blocking your way. Instead of bashing or running through it, we'll use the civilized way.
By having the key on your hand, slip in the key onto the door knob's keyhole.
Just make sure that it's inside fully.
And it's optional that you can make a 90 degrees twist counterclockwise or clockwise to open the door. If you still can't seem to turn the knob and open the door, make sure and recheck if it's the correct key, else you'll break it.
OMG- Thank you so much Internet! I've been waiting outside my apartment for days now trying to figure out how to get in. It's so simple! How could I have missed it?!