The link above is very useful - lots of good stuff in it.
As for your question, I am trying to figure out the same things! I've been at it for about 2 months now. Great fun. The mistakes I have made have been - duh - in my first purchases. So if you are going for the HOFers, I'd suggest buying some commons to get you started. Pick your favorite team or the cards you like the looks of and buy less expensive cards. That way you can learn what to look for. My mistake was not examining the scan closely enough and depending too much on the grade.
The other way starting with commons helped was I learned which grade to go for.
Reading is also good. The books 'T206 Collection' and 'Collecting Sports Legends' have been great and of course the excellent PDF Inside T206 by Scot Reader.
Good luck!
Eric
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