яд

Poison ...
я ел яд
i ate poison

somone decline this, i dont know where to begin

Nom + Accus. Яд - Яды
Genit. Яда - Ядов
Dative Яду - Ядам
Instru. Ядом - Ядами
Prep. Яде - Ядах

+ 2nd genitive singular. яду

All declensions have the first syllable stressed.