Also, if you have a 64 bit OS be sure you have the 64 bit version of Java installed, you can check programs and features in the control panel so see if it is there. I would recomend getting the latest version of Java 7 as well, then get the 32 bit and 64 bit and remove all old versions.
You can also just modify the launcher, seen below, click on the Edit Profile button
Then towards the bottom put a check in the box for JVM Arguments, change it to -Xmx2G if you like, that gives you 2GB of RAM for Minecraft. You can also increase it as needed, I normally run at 4GB....