When i go on in Minecraft I click Play but I crash! It happens alot please fix.

Put the summary of the bug you're having here

*What I expected to happen was Minecraft to open but instead crashes.
When I go on it says i have crashed! Yes, i have waited for a long time but still no, I always click play but i end up to crash.

What actually happened was when I get on hopeing to get luck too see if I don't crash but i ends up to crash.