If you didn't record to the metronome in the first place, the metronome's going to be useless due to natural speed variations while playing. You'll have to map the tempo envelope to your recording in this case if you want to hear it in time with your song.
https://forum.cockos.com/showthread.php?t=118061
You can find an approximate tempo for your already recorded song if you make a time selection that's 1 bar (zoom in, loop, look and listen) and then run one of the 'detect tempo' actions. You'll also need to open the Project Preferences and set Timebase to time before adjusting the tempo or you will change the speed of your recording.
If you want to start recording to a metronome (which makes editing 1000 times easier), start by clicking on BPM (it will say "tap" when your mouse is over it) in time with how you would tap your feet to keep the beat while you listen to a recording of it or "play" it back in your head. But some songs won't sound right recorded to a continuously even beat, so you'd have to go with the first method to accommodate the variations.