I have recorded an audio track with reaper and set it to record guitar but before I started playing the guitar made some noise and it was recorded.
How can I remove this?
I think you want to trim the audio file you recorded.
First you want to turn off snapping so that you can trim and not snap to the grid that you are using.
Then you hover the mouse over the front edge of the clip and you should get a little icon thing then you can click and hold then drag and drop the beginning to where you need it.