In other words, this site is pretty safe...
If you don't know what terminal is you should really avoid this. (the Terminal.app is located in the /Application/Utilities folder)
Spotlight is great and I honestly think you should avoid disabling it since it is one of the best feature of the system.
If your computer is running slow and you suspect Spotlight to be responsible for it, you may want to look at this other wiki post:
This said, if for any reason you still want to disable the spotlight indexing use the following commands from the terminal. Please note that sudo prompts you to authenticate as an administrator.
To disable Spotlight:
sudo launchctl unload -w /System/Library/LaunchDaemon/com.apple.Spotlight.plist
sudo launchctl unload -w /System/Library/LaunchDaemon/com.apple.metadata.mds.plist
To re-enabled Spotlight:
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
sudo launchctl load -w /System/Library/LaunchAgents/com.apple.Spotlight.plist