I just configuaring a new wifi modem but it just asking password for first time when I'm trying to connect my mobile why?

Modem's are used to convert the phone line into data line. Since they used widely in all countries, they are disappearing day by day, Routers came into existence. 

If you are installing the modem for the first time you need to refer router's manual first. First configure the modem in a way such that you can use it. Wifi modem is used to connect to internet through wireless.

Before Buying a modem check for the technical specifications like frequency speed in mhz and how long thus it spreads the waves, speed also matters here.

If you are connecting the modem for the first time, manufacturer of the modem will attach a username and password by default. May be mostly "Admin" & password is "password". Login using this credentials, if not worked refer your router manual.

Assign a WEP/WPA key for accessing Wifi, it is an easy way of maintaining the security. No one can access it. If you use the modem like open network any one can access it.

Make sure that maintain the security for your modem and internet aswell. dont share you passwords and usernames with anyone.