Here is the link of the site that I find the help.

First, you have to download the font you want on your computer. There are a lot of beautiful fonts on Google Fonts Choice your font, Download all. 
Extract your ZIP on your computer on a new document you will create. 

On your Dashboard, click Down on Settings/Files/Upload files (up in the page). Go open on your computer the Font file "ttf" from your computer, the font you want to put on your site. Once the file is downloaded, you can see an url link at the right of the file downloaded, "Copy the link", It will serve you later. 

Go on your Dashboard Shopify/Online Store, Actions/Edit Code/Snippets/Add a new Snippet/Create a new snipped called. Put the name called «Custom-fonts», then Create Snippet.
Put the following code inside:

@font-face { font-family: "Precious";
src: url('') format("truetype"); }

h1,h2,h3,h4,h5,h6,html,body,*,[id] *{
font-family: "Precious" !important;

1/ We have now to paste the copy link on the url code, the one we copy just before up 👆 

2/On your Pc, go to the font file «ttf» you just download, click right, rename, and copy the name of the file without the ttf extension. 
Then at the ligne 3 & 7 "font-family", put the name ttf  of your font, mine is "Precious" for example, change it twice by the name of your ttf font.

Save it.

For finish, go on your Dashboard/Online Store/Themes/Actions/Edit Code/Layout/Theme.liquid, tape CTRL + F, search "</body>". Scroll down at the very end of the theme.liquid file body. And just before </body>, put the following code:

{% include 'custom-fonts' %} 

Save it.

Your font will be include in the whole body of your theme!

Go check on your site, Voilà! :)