Selection from multiple option in a text field

I am creating a form where the user can select from a list inside a textbox.

Because I know very klittle of Javascript i have reched a dead point, and I hope to find a solution.

What I am trying to do is this: when the user clicks inside a textbox a list of words pops up, ex.:

Alfa

Bravo

Charlie

Delta

Each time the user clicks in the textbox the list pops up and the user selects one word.

Let's say that the user selects first Delta, then clicks again and selects Alfa, I want the textbox to show this:

Delta, Alfa.

How do i do it?