hey so your getting some new snowboard gear, be it new or second hand.
BOARDS you should find out all the dimensions of the board, mainly lenght & width, the length should generally be tail on the ground & the tip just at your chin, or find out the boards weight reccomendation. with second hand boards you want to get as many pictures off the seller as possible, to see the condition, look out for rail damage(on the edges) as it can cost a lot to repair, do a bit of homework on the brands & model of board you are buying, then get bidding & score yourself a sick new board for a bargin!
BINDINGS again check the size of binding & whether they are mens or womens, large bindings are generally a size 10us boot up, mediums 8-9.5 & smalls 7.5us & lower. with second hand bindings also you want to find out what condition the ratchets are in, if they have broken springs, can be really annoying. step in boots & bindings are sweet for learners-intermediate, but you probly want to get normal bindings when you get into the park(rails & jumps)
BOOTS these are generally the same fit as your skate/casual shoes, its always good to go into your local snowboard shop & try on the same brand of boots to make sure you get the size right, most boots fit into most bindings, you just have to be careful with flow brand bindings, not all boots fit in them well.
sweet well hope this helps with getting yourself a new snowboard set-up. go up the hill & have some fun!!!