How do I do that?

With the -Xmx1G? Becouse i already tried that i have 12 gb o RAM and i gave it 6 but this is what happens

-------------------------

Invalid maximum heap size: -Xmx6G

The specified size exceeds the maximum representable size

I think he is using Windows 64 bits but he has Java 32 bits.

If you have Java 32 bits the Minecraft crash report indicates that Windows is 32-bit even if you have Windows 64 bits.

 

http://stackoverflow.com/questions/1856565/how-do-you-determine-32-or-64-bit-architecture-of-windows-using-java

Please note, the os.arch property will only give you the architecture of the JRE, not of the underlying os.

 

If you install a 32 bit jre on a 64 bit system, System.getProperty("os.arch") will return x86

 

 

Luisggh : Go here : http://www.java.com/en/download/manual.jsp to download Java 64 bits.