As already mentioned, Orc and Goblin Shamans don't use the normal lores of magic available to human Wizards. Instead they have their own brand of sorcery known as Waaagh! Magic. With typical orcy simplicity, this is divided into spells of the Little Waaagh! and spells of the Big Waaagh!
Spells are chosen in the normal way as detailed in the Warhammer rulebook. Level 1 or 2 Shamans can take spells of the Little Waaagh! Level 3 or 4 Shamans can take spells trom either or both the Little Waaagh! and Big Waaagh! tables. Decide before you roll any dice how many spells you want to take from each list.