My car has been sitting in the driveway for approximately 3 months. I recently had it registered, topped it up with all the necessary fluids and went to start it. Started 1st pop, but when I went to put it in 1st gear, while the engine was turned on, It would not go into gear. But, every time the engine is turned off, I am able to put it into gear. Someone please help as this car is parked in my work driveway and they want me to remove it... yesterday.

PS, I am a female, so could you please speak in Lamens terms, thank you.

Silly question but... are you stepping on the brake pedal when you try to put it into gear? Pushing the button on the gear selector?

Stick shift? If the car has been sitting in wet weather, the clutch may have rusted to the flywheel. Sometimes it will free up with a push. Also make sure the fluid in the slave cylinder, if so equipped, is full and bled.

This sounds like a stick shift car with this type of problem. I could explain in very layman's terms if you would report back.