I highly recommend playing through AC, AC2 and AC:B prior to AC:R.
AC1 - Yes, pretty mundane and boring as far as the gameplay but rent it for a weekend from GF and play it for the story. If nothing else the introduction to Desmond (voiced by Nolan North. That alone!) The story kind of makes it worth it because you learn about the Assassins, Desmond, Altair, Abstergo, Lucy, and all this other stuff that keeps going through the rest of the games. Plus at first it's pretty fun until you realize you've done the same mission like 20 times.
AC2 - It's rare a Dev Team listens to people as much as the team behind AC does, because the second game is LEAPS above the first. The game controls better, is much more epic, expansive, so much more to do... and yes, they made the story even better! I dug Altair, but I admit I'm a bigger fan of Ezio overall.
AC: Brotherhood - Oh, you keep listening Ubisoft? The team does it even better. They scrunch down the locale a bit (instead of numerous different places you're in Rome) but kept the expansive nature, and used natural surroundings to break things up a bit (mountains, open areas, dense city areas, etc). So you don't feel like it's the same building 24,000 times. The story goes even deeper, and the last moments. They're the most WTF I've experienced in a game, and I've been jonesing to know how Ezio's story is going to end since!
Can you get away without any of the above? Sure. You can wiki it or whatever, but I don't recommend it. Desmond is one of my favorite characters due to his humanization and how he was just thrown into this, and it keeps getting better and better.
Also, as I mentioned, he was voiced by Nolan North who is one of my favorite game actors (also known as Nathan Drake for you PS3 owners).