The game crashes on start up

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here
The game to start up as normal

What actually happened was...:
Describe what happened here
The game crashes during start up and a Crash report is produced. The message commences with :
java.lang.OutOfMemoryError: Java heap space

I can't attach the crash report as it won't let me save it to disk (I did select the "Report to Mojang " option which brought up this template)

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
Starting up computer with no other applications running, select Minecraft

2. ... Minecraft messages of not responding displayed, game begins to load slowly then freezes
3. ...Crash Report displayed