I don't know anything about automorphic form but I heard it is related to representation theory and number theory. I am interested in both fields, so I would like to know if there is any introductory textbook or online paper that starts from the very basic.
I only know a basic of finite group representation theory and algebraic number theory (not class field theory.) So the more elementary, the better.
On
There's a lot involved in automorphic forms, and a lot of aspects to come at it from. It is good to learn modular forms or elliptic curves first, though most accounts of modular forms don't make the representation theory aspect evident. (The advantage of elliptic curves is that you will probably see the representation theory sooner.)
My suggestion is to read some surveys of what it is all about first. I have a short overview, and a list of open-access resources. The list includes some course notes and a bunch of survey papers. Gelbart's "elementary introduction" on that list is a classic. Don't expect to understand all the details, but you can get a rough overview this way. Then you can try to figure out what you want to learn in more detail.
You are going to want to learn something about classical modular forms first. A very approachable text for this is Diamond and Shurman's A First Introduction to Modular Forms.
Afterwards, perhaps the most friendly automorphic forms and representation theory text is Bump's Automorphic Forms and Representations. However, you mention that you are not familiar with class field theory, and this is going to give you some trouble. The reason is that the simplest case of automorphic representations corresponds strongly with the ideas from class field theory (and in fact, one might consider the Langlands Program a vast extension of class field theory).