Python | How fill website form and submit clicking a button

I am new into coding in Python and I can't find a good resource for the project I have in mind:

What the program should do, is select a reply from a drop down menu, fill a blank form with a phrase I have in a .txt file (there are multiple line, so choose randomly from them) and submit it (there is a button to click). Everything in a web page.

I don't know which module to work with; I thought it could be done with urllib2, but I don't know.

Can you suggest me where to look at, please?

Thanks!