Help! (piog)

ok.. so i have a 20 month old and almost 8 month old.. and someone has to help me with my toddler.. she is always screaming and throwing fits.. when she gets mad she will usually walk over to the kitchen throw herself down and hit her head on the floor.. or if she gets mad she will walk over to her sister and beat her on the head.. mix that with the damn blood curdling scream she does omg.. i just wanna shoot myself sometimes!!  so heres my issue.. if she does something majorly bad i will spank her like when she sat on her sisters back and jumped. she got her butt popped but honestly she doesnt understand it so i dont do it.. i cant smack her hand and tell her no hitting.. i wont do timeouts because she doesnt understand it yet either.. shes very very hardheaded and i have NO idea what to do with her.. she is driving me nuts somedays.. and makenzie is now starting to throw toddler size tantrums and i dont need her learning from her sister.. so how do i make it stop???? please someone help me..