You're going to need to download two different things. One is the MCPatcher (Found Here). It allows you to install mods. The other is the Optifine under the heading "OptiFine 1.0.0_HD_S_B2" or "OptiFine 1.0.0_HD_MT_B2" depending on what you want(Found Here).
Start the launcher, click "Options" and click "Force Update". Then log in. After the game menu loads, close it.
Installing The Patch
1: Open the MCPatcher (be sure to allow execution) and click Mods" in the toolbar. There should be an option to add a mod. Choose that and point it to the directory that you downloaded Optifine to.
2: Uncheck everything except "Optifine" and "HD Font". Click patch.
Once it is finished patching, launch the game. It should load fairly quickly. If it doesn't, you're going to want to retake the steps that I told you (from "Clean Installation") If it loads the game menu, click "Options" and then "Video Settings". You should see a whole load of new settings. Click around, and see what you like. It shouldn't take you long to figure it out. If you hover your mouse over any option, it will give you a description of what it does (I recommend doing this for every option). Hope this helps!