Hi :) There are many complete collections of his work but I would suggest buying one with selected poems at first and see if you enjoy them before getting more. Here's a penguin edition with selected poems by him. I used sparknotes a lot when I was in school to analyse old literature and they have a lot of stuff on Blake. Looking up the poem on sparknotes will provide an explanation of what he's saying, the poetry techniques used and how they help enhance the messages of the poem and analysis- they're really satisfying to read so if you are having trouble understanding or want to look at analysis then I recommend it!