I live in Japan but now I am in Nepal and I am not being able to use carriers of Nepal.Any help

You may need a SIM card from a carrier in Nepal. That requies your phone be unlocked from your Japanese carrier which may be difficult to accomplish.

Click on the link to read the article on "How to unlock your iPhone for use with a different carrier" - http://support.apple.com/HT201328 - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.

Which error do you see on your screen when you install another carrier's SIM card in your iPhone?

Axel F.