Why dose minecraft keep crashing?

Okay so i'm trying to download minecraft 1.8 from mineshafter it keeps crashing on me everytime i click play it will load then crash, i know its not my computer because i had leagues of legends and it ran fine, i have no idea what is wrong! Please help me! Anyone know how to fix it, also i don't have any mods this is my first time downloading minecraft

You may need to adjust your video settings, or perhaps you downloaded a virus by mistake.

Change your settings; if you're playing Minecraft on an old computer, you might have to change your settings (i.e: Fancy graphics->Fast graphics, etc.).

How do i do that?