This is not how you should use a custom font on a website. First, you should use an external style sheet and have all your CSS in that. Using inline styles is not a great practice. Second, I do not think you can link to a .ttf file as you want. Notice also that your code had wrong inline format. font-family: not =. Also, the whole inline style needs to be in quotes. style="text-align: center; font-family: 'Sigmar One', cursive;"
That being said - you could link your font in your 'head' of your document and then use inline styles to style h1. Here is a way to do it with a google font. Hope this helps!
<head>
<link href='https://fonts.googleapis.com/css?family=Sigmar+One' rel='stylesheet' type='text/css'>
</head>
<h1 style="text-align: center; font-family: 'Sigmar One', cursive;">Welcome to Ben's Minecraft</h1>