Help required on playing chords

I want to know when playing chordslike E or Am or any other do we have to hit all the strings at once or sperataly, If seperatly then in what order?
Please reply me soon

 

Hey plz um body reply i am having serious dificulties in playing in

 

U plz follow a chord book......
different cords are played differently.....
some chords r played open, some r fretted n some r muted.....
all depends on the cord Ur playin............