First find a solid site to download the game from. i recommend getting it from a freind but if none is too close the try minecraft.net, i've heard nice things about it.

NOTE: Minecraft takes up a lot of memory and its more fun to play it with a mouse and keyboard instead of the weird pad on laptops so be careful where you download. Also most schools have a policy aghainst these kind of games being played during school hours so watch yourself.

Once you have downloaded it you should have a folder iit was saved in. enter it and sometimes there are more folders inside to hack the game and stuff. if so clikc the folder labelled minecraft game and at the bottom there should be a block symbol labelled MinecraftSP open it, enter your name and create the world you want to play in (note the multiplayer option, you might use it later. also check your settings just to get a good look around).