I guess you have an audio track in Logic's arrange window
and you want to record vocals straight to that track.
If you click the record button on that track a save-as window
should pop up with AUDIO as the filename.
Click save, press record on the transport bar and record your vocal.
If this fails you may need to select and input for your audio track.
To do this double-click on the audio track to take you to the track's
mixer (you should see the rec button flashing on your track).
On this mixer panel, look above the volume and pan paders and
you should see an I/O section.
From the flip-down menu select your input source (eg. Input 1-2)
Press record on the transport bar and try again.
Hope this helps.
Let me know how you get on.