I got hacked

OH mY GOd
I got hacked

I even didn't know..Someone using my account
and left message to people...Everything that I wrote. is not what I wrote on replying...

I don't even write..to people and someone took my account and wrote..to people and replied..

Today I got tons of reply and I don't no idea..why people are saying something that i don't know about...
and I saw that I wrote ... I didn't write.....AT All

How to delete my account and I will never use again.??