Wont go in gear when on it only goes in gear when car off
It's a manual transmission, then? If it is, your clutch is not working. You need to check at the transmission's bell housing to see if the clutch pedal being depressed causes the clutch fork to move-either forward or backward, it should move about an inch with the pedal pushed to the floor.. The clutch pedal will either be cable operated, or it will be a hydraulic clutch with a clutch master cylinder on the firewall, and a steel tube going down to the bell housing, where a small "slave cylinder" operates the clutch fork.
If you see the clutch fork move a fair amount, at least an inch, then you probably need a new clutch disc and pressure plate. A clutch job means removing the transmission to replace the clutch.
If the clutch fork does not move, either the cable has too much slack-for cable operated clutches, or the hydraulic unit has a problem-for hydraulic operated clutches. One of your hydraulic cylinders may be bad, either the clutch master cylinder or the slave cylinder.