Preferably a chair shaped something like the chair you’re going to be sitting in during your lesson. A chair with a reasonably straight back that will allow your legs to rest comfortably on the floor.
I’d not given much thought to this, but lately several new students have had problems playing their best when they are seated in front of me. (Yes, I know, it’s a common occurrence, and not just with the newbies!) This is when the “I can play it fine at home” syndrome firsts rears its head. In trying to troubleshoot some of the possible reasons for what’s going on (other than “I’m scared to death!), it has come to light that these beginners often practice sprawled out on the bed. (Isn’t “sprawled” a great word?) Or in some other ungodly position that is impossible to replicate in a teaching studio. Like on the couch in front of the TV with one leg tucked up under them. Or with their stocking feet resting on the coffee-table in front of the couch. Or kicked back in a Laz-E-Boy-type chair.
Thus, when they move to an upright position their hands—both right and left---are going to be a slightly different angle. Things are not going to feel the same. And this one little change is more than enough to throw their playing off track. Hence the suggestion: practice sitting in a chair! And don’t slump!
That reminds me of my high school band director, Ann Alford. She taped thumb tacks—points out—to the backs of the chairs so that the clarinet section, especially Jimmy Holbrook, would have to sit up straight! (I just put Jimmy’s name in there so that if he ever Googles himself, he might find this mention in a banjo blog!) Thank goodness I was not a “real” musician in the band. I was in the color guard one year and played the bass drum the next. If I had a chance to do that over, however, I would most definitely play an instrument. First choice: snare drum. Second choice: that other drum. The one that’s not the bass drum. Tom-tom? After all these years I still want to be the loudest instrument in the band! Now you know why I play the banjo!