www.w3schools.com is a pretty good place to start
The tags that you listed <p>, <ol> and <li> are actually html tags.
The site I'm recommending has tutorials for html, php and many other scripting languages. I hope this helps. I'd definitely recommend becoming familiar with html before delving into the world of php. If for no other reason than to be able to know when you're scripting in html and when you are scripting in php.
By the way the w3schools web site will give you a jump start but there'll still be room to learn when you finish.
Also the php manual is here.