how to convert HTML control to asp.net controls?

First you should be add   runat="server" attribute for control and also provide a Id to that control.
Now you can access that control in aspx.cs page using id

Exa:
I want to convert a textbox to html server control



for this i will add a runat,id attribute to textbox  runat ="server" ,id="textId"



now you can use this cotrol at aspx.cs page using this id="textid"