RzenConcptS13

So i got out and i was out from work, i went and started my 240sx, everything was fine, im curious why i cant put it in gear when the car is on, but when the car is OFF i can put it in gear easily,and i wont let me go in resvere. my friend said master cylinder? or tranny? cars been very reliable to me , havent had a problem in the last 6months. and like i said im a noob knowing whats up, cause i bought this car as a daily driver even though ive put a little money to it. so help will be great.