Preface: this is the incorrect forum for this variety of post, in the future, please select an appropriate board.
I'm just elaborating on what veloticy said, but here are the proper steps. Allocating too much RAM or allocating RAM in incompatible increments is also a bad idea, not just for your minecraft, but for your PC as well.
1) Open your System window by pressing the Windows + Pause key. Check the amount of memory (RAM) installed in your system. Generally, it would be unwise to allocate more than half of your RAM to minecraft, seeing as obviously your PC and other applications such as the browser you are reading this in require RAM as well.
Proper RAM increments are as follows:
-Xmx2048M -Xms2048M - 2 GB
-Xmx3072M -Xms3072M - 3GB
-Xmx4096M -Xms4096M - 4GB
-Xmx6144M -Xms6144M - 6GB
Sidenote: Xmx is the amount of RAM that will be pre-allocated from startup and Xms is the maximum amount of RAM available for minecraft to use. There will usually be no need for more RAM than 4GB to be allocated to your Minecraft.
Here is the download link for Java SE, just incase you don't already have it installed. This will ensure that you are able to increase the memory allocation for Minecraft, which runs on Java.
http://www.oracle.com/technetwork/java/javase/downloads/index.htmlGood Luck!
-Azterix