To create a HTML5 is pretty straight forward. What you have to do is to include this specific doctype <!DOCTYPE HTML> at the beginning of your document.
The <!DOCTYPE HTML> type does not have to be uppercase, it can be lower-case, no restriction of case sensitive in HTML5.
See below example of HTML5 document.
<!doctype html> <html> <head> <title>My First HTML5 document</title> </head> <body> <h1>Heading content</h1> <p>The content of your website</p> </body> </html>