As a general rule on DAWs, you can just ignore the first bar or two and trim the beginning to where you want it to start: your recording doesn’t have to start at “zero”, and anything before you start playing is easily trimmed off.
The end result of your recording is the bit you export which doesn’t have to include anything recorded at the beginning or the end.
If you look at how they used to record onto tape in studios, they did the same thing: trim off the start of the tape before the song started properly. Of course it’s a lot easier to do in a DAW these days.
A lot of DAWs (not sure about Reaper) even have special “start” and “end” markers that you can use to indicate which part of the recording is the song, and will use this for export.
Cheers,
Keith