disciplining 18 month old!!!!!

how do you discipline your 18 month old??? my little boy is TESTING me every second. spilling my drinks... the dogs water bowl.... you name it. I work as a nanny and he comes along.. I watch a newborn and 20 month old girl as well my own child. so now she's beginning to test test test and wait for a reaction from me. HELP or I may just go insane!!!!! I show my son love and try and cuddle him and get down on his level and tell him no we don't do that..... but he wiggles out and continues being crazy! he's been acting out since yesterday. idk if I'm reacting to every little thing or what... HELP MOMMAS :)