When I started playing (20 odd years ago) I found Mel Bay's 'Fun with the Mandolin' book a good starting point for chord tables, picking styles etc. MB also published some books for more advanced Bluegrass methods.

The one bad habit I regret, is not using my left hand little finger enough, from the start. It is perfectly possible to play the mandolin using only 2 fingers on the left, 3 can be wonderful. But for the more complex tune 4 will be very useful. So work that little finger...

P.S. Don't bother with the left thumb, it'll only get confusing...

Have Fun.... Dave.

"Oh Lord! It's hard to be humble... But I'm doin' the best that I can".