Focksbot

Hi -- I'm merging a site with a pre-existing one that uses ProcessWire, so just trying to get my head round it. I'm used to using WordPress, or writing my own sites from scratch out of a mixture of html, php etc. Sorry if this is a stupid question, but where can I actually insert my own code or start making my own parts for a page? The pages are made out of templates, and the templates out of fields, and the fields seem to be of a number of very limited types. I can't work out how to customise anything at all in the way I'm used to.