When I started taking "live lessons" I could play a little bit and the first thing my instructor told me was thumbs up. You'll need it later on to mute the top strings. The only time I drop my thumb on the back of the neck is when I have too. IE barre chords,power chords, and long stretches(hmm which come to think of it is most of the time) so both ways are needed. The easiest barre chord I think would be Bm I would just keep moving it up and down the fret board till I got the hang of it(obviously once u move your barre off the 2nd fret its no longer a Bm) . chris has some lessons on this which should be helpful http://www.guitartricks.com/lesson.php?input=11253&c_id=2&ch_id=17&s_id=450