First of all, lets recall what is title tag in html.
The title tag is a section of HTML code that every page should have. It declares what the page’s title is. It also defines a title in the browser toolbar when you hover a mouse on it and provides a title for the page when it is added to favorites. The more important that it's displays a title for the page in search-engine results. I'll give you a bright html title example of this page:
<title>How to make TITLE meta-tag correctly in html? | Captcha Forum</title>
What do you mean about "correctly" writing a title? It's up to you to decide, since you must know better what is good for your visitors - what they are looking for and what words will be vital for them.
Still, it's a good idea to put some high frequency keyword in it.