How to use my iphone when I forgot password of find my iphone? Please help me.

If you mean that you used the Find My iPhone app to set the device as "Lost" and put a password on it, and now you have an activation lock..

Read through this article to find if your device has an activation lock.

http://www.macworld.com/article/2048739/get-to-know-ios-7-activation-lock.html

If this is what you have - the news is bad:

http://www.cultofmac.com/246755/why-ios-7s-activation-lock-is-a-disaster-waiting -to-happen/