Welcome to the Reaper Forums!
In case you haven't already found them, make sure to check out the User Guide (
https://www.reaper.fm/userguide.php) and Videos section (
https://www.reaper.fm/videos.php) in general.
You can change the Track Input here:
In general, also make sure that the input range (from first to last input) is set correctly under Preferences > Audio > Device.
More info in chapter 1.12 in the User Guide.