I remember when I started, I was so eager to fly I was using (to get to the radio) esc key, clicking radio on the clipboard, then clicking a channel, I also didn't know about the check 6 key shortcut, so calling out a bad guy who was lining up a friendly was a panicked flurry of movements!
Luckily Ghosth and Ozark were very helpful in my first few weeks and helped me out with it.
As SIK says, / will bring up the radio, and you'll be on country (green) channel, depending on what you have your other radios tuned to, shift / will get you the second channel, ctrl / the third and alt / the 4th (I think, I might have the order wrong, it's muscle-memory for me now)

Welcome aboard!
Salute
Steely