Before uploading a logo, your header will automatically display the text of your store's name as set through your Shopify admin.
Alternatively, you can replace this text with an image of your logo by going to Header > Upload logo image > Select or change image.
To add an image, you have the option to Upload a new image, or to select an image from your existing Shopify image library (as stored within your assets folder within your Shopify admin, which can be found under Content > Files).