Start off with a Doran's Shield for a tankish Garen from the beginning. After a little farming blue pill and get some boots. Do some more farming and get the Boots of Swiftness and a Giant's Belt. Next buy the Sunfire Cape, which is necessary for all tanky-dps champions. Guardian Angel next will help you to stay alive and sometimes lead to the delay of the enemies your teammates need to get to your position. Thornmail next if the enemies are mainly physical or Force of Nature if your enemies are mages. That should be enough defense so buy Atma's Impaler for some extra damage and armor, then Warmog's Armor for incredible health and even more attack with your Atma's Impaler.