Dark Souls is a fantastic game that I'd recommend to anyone looking for that kind of challenge, but you don't need to play it before Dark Souls II. Playing Dark Souls beforehand probably helped me die fewer times in Dark Souls II and mutter things like "WTF happened to the Estus Flask?!?"
I highly recommend playing both games with a controller. I've never used a mouse and keyboard for either game, but I've heard Dark Souls 1 is a nightmare without a controller — in a bad way, not "BUT MASTERY OF THE MOUSE AND KEYBOARD LEADS TO GREATER SATISFACTION AND SPLENDOR" kind of way.
Finally, download DSfix if you end up playing Dark Souls 1 on the PC. I've only played the PS3 version of DS, but I've heard that mod is required if you want an enjoyable experience. The PC version of Dark Souls II just works from the get go, so there's that at least.