Assuming you mean Bluetooth, then it will be something like:
Settings .. Bluetooth
Then tap where it has your phone with something like 'Only visible to paired devices'
Note that the precise options may be slightly different depending on which phone you have and what version of Android it is running. If you can't work it out, try re-posting with this information.
If you're not referring to Bluetooth visibility, try reposting with the above information and more detail about the problem you are trying to resolve.
