You don't need to convert it as such, just change the file extension. I prefer Notepad on Windows for this. The reason you can just change the file extension is that PHP is written in HTML, except for the special programming bits inserted in it every now and again.
Giving extension as .php is ok but what all php codings come on the top and at the bottom to do it right? newbie
Keeping your php code seperate from the html makes it far easier to edit at some future point but it is not an abosolute requirement. One thing you should consider as you make the conversion is to make your header, footer, navigation and any ads a seperate page and then include them on your main pages. That way when you edit one header page the changes take place on all your pages. If you change from adsense to amazon you just edit the pages with the JS and it changes on all pages.