Hello and welcome to JustAnswer!
Can you turn the steering wheel in both directions, or is it locked?
It sounds like your steering wheel is locked and it's bound up against the linkage in the steering column so the key won't turn. This happens if you are holding the wheel turned when you shut the engine off.
Grab the steering wheel and try and turn it each direction. There will be a mechanical lock in one direction, but it will be able to move a bit in the other direction. It will be very stiff since the engine isn't running and you won't have power steering, but it will be able to turn in one direction or the other. What you will need to do is turn the wheel with one hand in the direction it will go, and then turn the key forward with the other hand and the column will unlock and you'll be able to start the engine. If you aren't able to get the wheel to turn on your own then enlist a helper either to turn the wheel or to turn the key while you use two hands on the wheel.
Were you able to hold the wheel off the lock and the key still woudln't turn? If that's the case then you have a bad ignition lock cylinder, and I can walk you through that too.
If you weren't able to get the wheel to turn then you're going to need a helper to turn the wheel while you turn the key forward. It's going to be very stiff when you try to turn it since the engine isn't running, but it will turn in one direction with enough force.