To add a login in WordPress on Your website, please first login to your website by going to the domain name of your website followed by /wp-admin/  the the URL/Link would look something like this:  https://YOUR_WEBSITE_DOMAIN/wp-admin

From here you will see a screen that looks something like this:



Once you login,  click the menu item “Appearance” in the left menu.


Then click the sub-menu “Customize”.


You will be taken to a different area with a bunch of different settings for the design of your site.
Click the down arrow next to the “SITE LOGO & TAGLINE” menu option.  This will open it to give you more options.


Click the + (plus sign) next to “Site Logo” on the menu.


You will now see a bunch of options.
From here you can choose to use an image for a logo by clicking on the “Logo Image” option.


A new box will appear, click the + (plus sign) in the box to upload an image. 
It is recommended that the images you use are scaled already to the right size before uploading them.


If you choose to simply modify the text you can do that from this screen as well.
Just leave the first option “Site Title”, then chose the font, sizes, color and other options of your choice.