Bring up "Activity Monitor" by typing that into spotlight. Once the application is up, set the drop down list box (or pop-up menu) at the middle-ish top is set to "All Processes". Click on the "%CPU" column heading to sort by CPU percent, best to sort it so the highest CPU items rise to the top.
Look and see if anything is using CPU when the computer should be idle. Using CPU means like 20% or more when you have asked the computer to do nothing. Any such processes should be looked into with a thought to turning them off or removing them. High CPU utilization will cause more heat.
Additionally, the heatsink that the internal fan blows over to cool the CPU and GPU may be clogged with dust. Go to Radio Shack or some such place and buy some canned air. Blow the air into the vent at for a few blasts, you may be amazed at the dust that comes flying out. Don't do this too long as it could cause the fan to spin faster than it is designed to spin. A few intermittent blasts are better than one long one. If it is really clogged a professional opening and clean may be required, if you do not feel confident enough to disasemble by yourself.
Hope this helps.