To add a logo to your website, go to the Design tab > Logo.
Here you can upload your logo, or set a text logo if you do not have a logo.
You can set the size (unless it is pre-determined by your template)
If your logo is a text logo, you can change the colour, size, font etc and what the text says.
If you are more of an advanced user, you can make a logo up with snippets here as well.