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

<input type="text" value="find" />

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

<input type="text" runat ="server" id="textId" value="find" />

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