Also fifteensquared.net ("225")!
Try a puzzle each day and compare your work with the annotated 225 solution when it appears.
That gives a lot of insight into clue types (e.g. hidden, double definition, charade, cryptic definition, anagram, &lit, first letters, etc.) and thought process.
Sooner or later you'll be solving and find yourself saying "there's a hidden! (or whatever)".