they are in the item shop right by the health and mana potions
however at the level you're playing at you don't have to worry too much about other lanes coming down to kill you
focus on learning how to last hit first! it's one of the most important tools in league of legends. basically you try to get the last hit on every minion in the wave since you only get gold for minions that you kill, not minions that your minions kill. it's about timing and it's easy enough once you get enough practice.
for item builds and what champions do, use solomid.net, lolpro.com, or the top rated guides on mobafire.com. the first two have guides written by pros and are the most reliable but mobafire has a TON of information on every champion and is much more helpful to new players.