There is a post on the front page of this forum in which a lot of vets have given their opinions on when to begin drafting IDPs.
You can find it here
Without getting into a whole lot of detail, player values usually break out along these lines.
4-3 DE > stud 4-3 undertackle > 3-4 DE > 4-3 DT >> 3-4 NT
4-3 MLB > 4-3 WLB/3-4 ILB > 4-3 SLB/3-4 OLB
4-3 SS > 4-3 FS/Cover2 CB/3-4 SS > CB
Depending on a particular player's skill set and particular defensive scheme and your scoring system, though, there is a lot of variability in the above generalities.
In general, players on the strong side (except the strong safety who's a level removed from the fray) see more blockers and less opportunities to make tackles.
Ask plenty of questions, use the search function, and play around with your league's scoring system. You'll pick it up quickly.
John Norton writes a great article every year about the fantasy value of each position. I don't remember if it's subscriber content or not.
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