My Minecraft won't start

I can get as far as the minecraft launcher but when I press play I get the following message:
Exception in thread "main" java.lang.NoClassDefFoundError: net/minecraft/client/main/Main
Caused by: java.lang.ClassNotFoundException: net.minecraft.client.main.Main
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)

I don't know what to do, and I don't know anything about computers or how to fix them. please help. Also I don't know what "Affects version/s" is so I put whatever.