Is "manners" a singular word or plural?

"Manners" is always plural: "His manners are always impeccable"; "good manners are important to us".

You can say "He has a good manner" - but that does not mean the same as "He has good manners".

plural and i don't think you can use a in front

It's plural and you can't use 'a'. It has to be 'Have good manners.'