Picking and fingerstyle techniques

It depends on what you want to do. For most fingerstyle playing, you use the thumb for the three bottom strings ( bottom meaning low E, A, D) and the index, middle and ring for the higher strings. There are exceptions to every rule, but this is generally how it goes.

Hybrid picking involves more variation. I'm working on"white summer," which uses hybrid picking and you use the pick on the bottom four strings and the middle and ring fingers on the top two. But it all depends on what seems to be the most efficient. I've played some blugrass stuff in open G where you use the pick on every string except the high E, on which you use the middle finger. I don't know about the body mechanics of it, but the middle finger makes it "easy" to cruise right along when you pedal with the pick ( by that I mean you play a note on the G string (for example) then play the high E, then back to the G, etc.) . Sometimes you'll have no choice but to use the ring finger on the high E and the middle on the B only, it depends on the song.

Hope that helped!
M

I've read and heard a lot of stuff from different sources about picking (with or without a pick):

-Use as many finger as possible (four fingers for the four highest strings, thumb for the lower two)

Most people pick with three fingers, rather than four. Your pinky is so much shorter than the others it's hard to get a good hand position if you're using it.

Fingerpickers tend to dedicate fingers to strings - usually the index gets the G string, the middle gets the B string, and the ring gets the E string. The thumb gets the E, A, and D strings for bass notes.

Classical guitarists also use the thumb and three fingers, but there are no string assignments. That takes a bit more practice.
-Two fingers and a thumb are all you need
The thumb and two fingers style was popularized by Merle Travis, and most folks call it 'Travis picking'. One big feature of this style is an alternating bass with the thumb. You can get some pretty nice sounding patterns with it.
-Use a finger (pinky or ring) to rest or anchor your hand on the guitar (near the sound hole, below the strings), so that your hand position is relatively stable and it's easier to find the strings.
Some folks do, some don't. Classical guitarists never anchor - touching the top really affects the tone of a classical guitar. When you see how a classical guitarist holds the instrument, the forearm rests on the edge of the lower bout, and the wrist is several inches above the guitar's top. That keeps you from inadvertantly touching the top.

For fingerstyle, it's a matter of preference. It can be helpful in finding strings, especially if you're doing something like ragtime.

One other popular fingerstyle technique is the "Carter Scratch", developed by Maybelle Carter. The thumb plays the melody on the bass strings, the fingers strum the treble strings.