I agree, start with Digger's eBook, it's a one-stop PDF file for all the basics about the entire X-Terra lineup from the day it was released up until the current models.
I have the 705 but the 505 and the 705 both have 5 channels of noise cancel. This really helps out with EMF from other detectors, power lines, electric fences and other things.
When I first started out with my 705 I put it in 4 tone audio and after getting used to the machine I put it in 28 tone audio. The 505 has 1,2,3,4 and 19 tone audio, start with one of the lower audio settings with the # of tones then move up as you learn the machine.
The biggest things are going to be setting the discrimination pattern, noise cancel, ground balance, and pinpointing. The 505 has three patterns available with 19 segments. The 505 has 50 settings for ground balance and the ground balance is manual.
The owner's manual has clear instructions on how to operate your X-Terra. The back page of the manual is perforated so you can rip it out of the manual and bring it with you detecting so you can use it like a cheat sheet. You can bring it with you as it is or you can get it laminated to protect it from moisture.
The order of operations is easy:
1) Turn it on
2) Only one detecting mode - Coin & Treasure
3) Choose a DISC pattern
4) Adjust Noise Cancel
5) Set Ground Balance
6) Commence Detecting!
One afternoon with your X-Terra and you're going to be amazed how much performance is at your fingertips considering it's low weight and the fact that it only uses 4 AA batteries. Best of luck!