I have a Galaxy Young phone and a 3g connection. I use my phone like a wireless modem but I don't want other devices to find my WiFi hotspot when they search for WiFi.
How can I stop other devices from seeing my hotspot?
What you're asking for is the ability to hide the SSID. Depending upon your phone and service provider, you may have an option to configure "Hide my device" for the hot spot. My Samsung (with Android 4.4.2) has that option and it appears to work. However, there seems to be plenty of other phone/provider combinations that don't appear to provide this feature, or, they don't work correctly as evidenced by this google android issue tracker.
Don't waste your time trying to hide your hot spot - you can't really.
What you CAN do however is to require a password to allow a connection TO your hot spot.
In some cases, not sure here as I do not have your device, it is possible to lock the hot spot and only allow connections from one (or more, if there are others who also need to connect) specified MAC addresses only.