I've been boarding since the early days, I think I started around 89-90..
Stick to the intermediate slopes, No beginner(too slow to have any stability) No Expert( too much speed & usually moguled out)
FORGET THE LESSONS... spend that $ on a another day riding back to back days, you will gain tons of knowledge just by being able to put to use what you learned yesterday..
ACCEPT the SPEED thats the other key, most beginners can't compute the accelleration factor of a snowboard.
Also don't worry about what length the board is just make sure your choosen stance is comfortable & stable feeling. general things about length hold true.( Longer = faster & stable , Shorter = less stability but easier tricks )
but are unimportant at your level as long as it's medium in length 150's -160's your all good
Enjoy it while your young and your joints can take it.. I went boarding a few weeks back after we got that good dump..My hip & knee was killing the next day
My prime days at Bear & Summit back in the day where well worth it !
Have fun.