trip I'm new to ereading too but I found that googling the reviews on the different brands was the best help to me.
Across the board it seems that kindle is the most recommended and Amazon have the largest collection of books. Kobo seems to run second to kindle in the size of their ebook collection, but as HarryT says, you can convert the books to a format compatible with your reader, so that's not really a problem.
I ended up with a Kobo Touch due to it's affordable price and because it has a touch screen. Also it was available locally thru a major store (I always buy electronic equipment this way rather than online so that any service/warranty issues can be cleared up quickly, without freight costs). File conversion via free Adobe Digital Reader is a plus, and the Touch had GLOWING reviews... which, as reviews go, is a bit of a rarity, so I felt confident that this ereader would do for me. The Kobo Glo was another possibility but it had terrible reviews so I sacrificed the luxury of an illuminated screen for something more reliable.
I'd highly recommend you browse reviews on the different brands-models to find what suits your requirements
