Since you don't mention it, I'll start of with 2,4GHz.
The choice of 20 or 40 MHz depends on your neighbors. If there's a lot of 2,4GHz around you, stick with 20MHZ as the AP will revert from 40 to 20 if it detects interference.
If you can go to 40MHz, then scan what's around you and select a channel (1, 6 or 11, for North America anyways). The above / below will become obvious once the main channel is selected. Since you can't go outside of the legal range, you'll choose the secondary channel based on the primary. So, if you choose channel 1, select 40Above. If you choose 11, select 40Below. That way, the secondary channel will always be within the "legal" range.
If you use 5GHz, you'll see from Ceee to eeeC (with the 80MHz bandwidth). The letters mean (C)enter, (e)xtended. Same thing as 2,4GHz, just more channels to consider. So, depending where you are in the 5GHz spectrum, you'll select your main channel and the extra ones always in accordance to country regulations and available channels around you main one.
Alain Casault, Eng.
If I helped you, let me know!