// Referring External Javascript ClientScriptManager cs = Page.ClientScript;
// Include the required javascript file.
if (!cs.IsClientScriptIncludeRegistered("jsfile"))
cs.RegisterClientScriptInclude(this.GetType(), "jsfile", "/_layouts/MyWP/jsfile.js");

Test :
Testbutton= new Button();
Testbutton.Text = "Click me";
Testbutton.OnClientClick = "jsfile_Function()"; // specify function name here
this.Controls.Add(Testbutton);

// Refering External CSS
Microsoft.SharePoint.WebControls.CssLink cssLink = new Microsoft.SharePoint.WebControls.CssLink();
cssLink.DefaultUrl = "/_wpresources/MyWP/1.0.0.0_9f4da00116c38ec5/styles.css";
this.Page.Header.Controls.Add(cssLink);

// Using External Image
imagePath = "/_wpresources/MyWP/1.0.0.0_9f4da00116c38ec5/Image.jpg";
img.ImageUrl = imagePath;
img.ID = "image1";

this.Controls.Add(mybutton);
this.Controls.Add(img);