This is a simplified version, there are plenty of detailed calibrations threads on here and other home theater sites. But here's what you want to do...
Get a dB meter, possibly from RadioShack.
Find the a test tone generator with pink noise, either built into the receiver or on a disc.
Start with the left, main channel. With its trim set to +0 turn the main volume control up until the test tone is being measured as 75 dB from your primary listening position.
Now go through the rest of the channels without touching the main volume, only adjusting their trim levels up or down to get them to also be playing the test tone at 75 dB.
Finally with the sub, set the receiver trim to 0 and use the gain knob on the back of the sub to get its playback to 75 dB.
