How to allocate more ram

In order to allocate Minecraft more RAM/Memory simply open up ATLauncher and go to the Settings->Java tab and change the amount of Maximum RAM/Memory using the dropdown, remembering to click the Save button when done.

If your only option is 512 and 1024 then that means that you only have a 32 bit operating system installed or you don't have 64bit Java installed. Please click here for ways to resolve this issue.