Ahh glorious Minecraft. I haven't had one of my Minecraft phases in awhile but when I do man oh man. The Lego comparison works well in the sense that you've got a giant bucket of random Legos and (in this case lets assume you don't have instructions for a kit) you need to come up with your own idea.. There is no ultimate goal other then the one you make for yourself so if you can't do that then the game isn't for you.
Also the Xbox 360 version is inferior to the original PC version so I would not advise in getting that. Not only is it a few versions (and features) behind, but you can't mod it which is a huge part of the game. Texture packs and mods that are essentially expansions adding systems equivalent to electricity, steam power, all manner of machines, magic, RPG elements, nuclear power, computer programming, solar energy, alternate dimensions, oil and fuel processing.. the list goes on and on. The game is pretty basic without these additions.
And playing it online with a friend or two makes a huge difference.