New (old) guy trying to learn Mandolin.  When playing by ear, what do you do with the left hand (I know this sounds crazy) but I know 3-chords C,G,D.  I can pick out notes pretty good with my right hand, but not really sure what I need to be doing with my left hand. If this makes any sense to anyone please give me an answer.

Thanks

well, you have to pick the right fret to get the note you want.

Before you try picking out tunes, it might help to play a few scales to get the feel for making different notes.

Here's a link to G scales--
http://www.mandozine.com/music/tabledit_files/Scales-G.tef
Just try the first to measures (the boxes labelled 1 and 2).  The top part is music notation which you have to learn but the bottom part is the same thing in "tab" which tells you which string and which fret.

You'll need a program called TefView to read it but it's free--
http://www.tabledit.com/tefview/download.shtml



Then, try picking a string with your finger on different frets to figure out something like Mary Had a Little Lamb.

If you don't have a teacher or mentor, you really would benefit from at least a very basic beginner book.