This thread explains how you have to post bugs in this section.
1. A common sence titel
How? Minecraft gets a blackscreen after joining the server
How to NOT do it: Minecraft gets Blackscreen :d
2. Error log
Start minecraft with a batch file that contains this:
java -Xmx256M -Xms128M -cp Minecraft.jar net.minecraft.LauncherFrame
And you should get a commandlog window.
3. You can post a video too how it happens, but i think you'll not need it because the most problems cause a Blackscreen. Post a Video if ther're graphical glitches or K.I failures.
4. Dont post: I GOT THIS PROBLEM TOO!!1111wtf! Post how to fix the problem! Do NOT post the same errorlog in the thread either.