I'd just look at the shelf for a while and figure out which range of models you like the most. All the armies are good, in the end, despite what people may say. The only way you'll ever finish the army, be motivated to play with it, and actually enjoy playing with it is if you find the models you like the most.
From a mechanical standpoint, it's probably easiest to learn the game using Space Marines. They're quite forgiving, and follow the core mechanics of the game more than a number of other armies that come to mind. There aren't really many special rules to remember with them, and you can easily switch between a ranged army and a close combat one with them, so you can find out which you prefer without spending money buying two armies, essentially.