1. Ask about the loot rules. You should ask the party leader about the loot rules ASAP. Yes, most of the time it will be need or greed, but sometimes it isn't, and I guarantee someone else in the group is wondering about the rules as well. It never hurts to ask, and by speaking up you will be 100% sure what to do when the loot roll boxes pop up. Sometimes groups will choose to go with Master Looter on bosses instead of rolling, and this should be established before you enter the instance.
2. BOE Items. Most items that drop off of the trash (non-boss) mobs will be Bind on Equip. What that means is even if you win the roll on that item, you are still free to trade it to others or sell it in the auction house. BOE loot windows are gray in color and will not prompt you after you roll to confirm that you really want to roll on the item. If you happen to win something that someone else in your party claims to need, it is good form to let them have the item. Whatever small amount of gold (or in the lower levels, silver) that you might receive by keeping the item will not be worth the bad reputation you will gain by hoarding loot.