Along with the manual, you can check the review at dpreview here. It has a lot of information on what the different menu options do, and why you might choose certain ones.
For simplicity, I would set the AF to the center point and leave it there. The 7D has a very complex AF system, one that takes some time to master. I borrowed a 7D from CPS for two weeks and was totally confused. For long lens work, the center point will be fine, and it's the most sensitive.
As Bryan mentioned, the control buttons are sticky, so it's possible to change settings inadvertently without even knowing it. Something to keep an eye on.
The 7D is a nice camera, and will do a good job with the 300/1.4x combo. Have fun with it.