template modification inquiries

Hey guys! I have just joined the Xenforo community and now trying to do some modifications of the default template!

However I have encountered a key problem that I have found where the files are but I just don't have a clue about how to change them! My previous site was on phpbb,so it was pretty easy to modify the looks and appearances, but seems there's great difference between these two.

Can anyone please be so kind to recommend some tutorials on how to efficiently modify the template?

Thank you!
Thanks for that! I will start with that. I am trying to do a template like the Metro style, a little bit of Win8 style in it. Any clues about it?