You will need to have your fonts converted to .WOFF (the Web Open Font Format). You get them here: myfonts.com.
Note. When making the purchase on my fonts.com, choose the WEB license so it can be used online.
Upload your font files online by choosing any server or CDN. Next, create your CSS file and add your font styles into it.
Learn more about fonts in Tilda: https://help.tilda.ws/fonts