I want to send SMS using asp.net with C#, How i can do this.

For sending smses you need a gateway.
That costs money.
For usually the gateway has got a controllpanel.
Sorry but I don't know much about mobile-gateways

shadow is right, you need a gateway.
Now, is whether you manage your own gateway (means your own hardware) or simply use thirdparty sms provider then you send the sms.

having your own gateway is troublesome, you need to have the hardware like gsm modem, the driver to send the sms. You can see swiftsms software to do this, they provide api for you to send using your own hardware or their gateway.
see http://www.swift-sms.com

However, i would suggest you using thirdparty gateway such as mobile365 and connect to their api simply using tcp ip, the easiest is just using post to their http url.
you can see them at http://www.sybase.com/mobileservices/