1. Your eagle is most likely worth around $1,000. 
2. I would not recommend third party grading (i.e., certified holder) without discussing your coins with a dealer to see if they merit the expense and wait. An A.N.A. trained  professional such as myself can tell you grades.
3. You mean post mint damage. 99.5% of the error inquiries we get are in error themselves: post mint damage mistaken for errors. I suggest you forget errors for now, unless you inherited labeled ones. You won't be likely to find them in common circulated coins (or any other coins of value, for that matter). 
4. To learn about these things (errors, authentication, grading, value), I recommend you read books not Internet sites (except the tutorials at the top of our home page, which we have written very carefully); join a club and go to shows; join the A.N.A. and read The Numismatist; and visit coin stores and talk to dealers. 

If you REALLY want to learn, go to ANA seminar in Colorado springs and take courses with us every summer!

Welcome to the hobby