Have you read the Common Problems sticky? Read through it, if none of those solutions in there help you'll have to edit your post.
Operating system is either Windows (7, XP, 8), Linux or Mac. Find out which one is yours
Java version, google if you don't know
Antivirus, tell us which one you have installed. There are a few AVs listed in the Common Problems which causes problem with the launcher.
Error log, there is always a log. click on that cog button on your top right corner of your launcher window. Then you will see the log button, clicking it will lead you to the log files.
Post the content inside
if it's too long, post it in pastebin and post the link here