It depends what kind of app you want to remove. It is something you dowloaded, you can go to settings>applications>manage applications and then go the app you want and click uninstall.
You can also download an app uninstaller from the market.
If you asking about pre-installed apps (e.g., sprint bloatware)
You might find your answer here:
http://forum.xda-developers.com/showthread.php?t=694263