How to navigate over the web using Python?

I was wondering what is the best way (i.e. the easiest one) to navigate here and there over the web?

I mean web scraping, but not only, I'd like to code a bot or a script able to interact with a website as a human would do with Firefox, such as create an account.

What modules do I need? It is required to be a killer in HMTL/XML/CSS?