This is a rough outline of things I do most games as support. If you are clueless about how to play support then following this will set you on the right path. After you do it a few times you'll start to work out what is going on, and should diverge off it as you see fit. This is for hard support, i.e. farm position 5.
Farm priority is a system to designate who gets farm when more than one player is in the same place.
Each player has a number: the lowest number gets the farm. So 1 is the hard carry in the
safe lane, 2 is mid, 3 is the offlane or jungle, 4 and 5 are typically
supports. The 4 saves up and gets Mek, while the 5 buys wards.
Start items I buy for practically every support hero is: Courier, Wards, Tango, Salve, Clarity.