Ummm...just find any site that describes proper HTML syntax.
Briefly, HTML pages must be laid out like this:
...meta tags, etc, here are optional...
If you use a <!DOCTYPE> (which you should), then the simplest to use is <!DOCTYPE html>
. Doing so adds a couple of other restrictions on your HTML, but the w3c validator will tell you about them.
Note that you can have only *ONE* <body>...</body> pair.
Note that if you use a <form>....</form>, you can *NOT* next another <form>...</form> inside. (You can have multiple <form>s, but they can't be nested.)
There's more, but that will get you started.