Help!! my employer is slandering my name at work and accusing of not doing my job what do i do????

i have worked for the same company for almost 8 months now. everything was perfect until i started making suggestions on how they could operate more smoothly. i HATE their business practices but up until now i have kept quiet. now they are threatening to fire me for not doing my job which i am doing!! my work hasn't changed at all in 8 months and i am doing everything they are asking me to do!!! they actually are making up things that they say i dont do. now i hear from all the other employees (which i am all friends with btw) all the nasty things my mgr says about me behind my back and tells people that i don't do anything, i look bad in my clothes, i'm a terrible worker, ect. and when i ask him about it he completly ignores me!! i want to know what can i do about it and if i can take legal action against them for haring me!! i am doing what i am supposed to exactly how they told me to do it1 please help!