Go-around is as SNS3G says, anywhere from the ground up. If you land too fast and balloon a well-executed go around will save your bacon and prevent the nosewheel being broken off.
However until you are very comfortable with the handling it's wise not to push the envelope too far. Pushing the throttle forward and retracting the flaps with one hand can cause the other hand to move, particularly if you don't have the trim correctly set and are holding on some pressure, or if the a/c has a big pitch change when you do it.
The right point to go around is when you realise that you are not going to land. Continuing the approach when it's plain as a pikestaff that the other a/c won't vacate in time is pointless.
Not unusual at larger airports for ATC to give you a VERY late landing clearance
They're masters of the art of holding the microphone open "Speedbird 243.................. continue approach................wind 240 10 kt.......................justwaiting for the previous aircraft to clear the runway...........clear to land!"