Website log in auto fill

I want to auto fill in a web page and then submit the data in C#.

How can I do this?

You will need to fake a POST HTTP-Request to the webserver. You will need the following classes: HttpWebRequest and HttpWebResponse. The POST data will be the values for the form fields, to find out the POST request a browser sends out, have a look at Wireshark.

Another way could be the WebClient.

Do you mean programmatically or inside of an actual browser window? If the latter, try WatiN.