How would I make a button, that on click it receives an HTML file or txt file to be implemented into a textbox?

From where are you retriving the html or text file? from the web? from a local file?

From the web...

Use the Internet transfer control and its OpenURL Method.

For example if its name is Inet1 and the textbox name is Text1 :
Text1.Text = Inet1.OpenURL("http://www.microsoft.com")

If you have a ftp access it works too :
Text1.Text = Inet1.OpenURL("ftp://ftp.microsoft.com/disclaimer.txt")

Use the Internet transfer control and its OpenURL Method.

For example if its name is Inet1 and the textbox name is Text1 :
Text1.Text = Inet1.OpenURL("http://www.microsoft.com")

If you have a ftp access it works too :
Text1.Text = Inet1.OpenURL("ftp://ftp.microsoft.com/disclaimer.txt")

What dou mean by the name Inet1? The name of the file or, something else? And what is it :p

In this example, it's just the name of the activeX control you have to add in your form or in your module (Internet transfer control which will permit you to use the OpenURL Method). The name of the file you want to get (Or the web page) belongs to the URL parameter of the OpenUrl Method (See example with ftp).