Pretty broad title, however, this should work.
To add a moderator, you must have admin rights. If you have this, navigate to your local Admin Dashboard. Hit "user rights" under the "Community" subheading. Enter the username you want to promote and hit "Load User Groups". Scroll down to the small boxes, and check off the ones you want for that user. View what each user right does here. Add a reason for the change, and hit "Save user groups". After that, you should see a pop-up in the right-hand corner, saying that the user rights have been saved.
Wordmarks also require admin rights. If you have this, navigate to your local Admin Dashboard and hit "Theme Designer" under the "Wiki" subheading. To use a wordmark, click the 3rd tab on the bottom, that says "Wordmark". Pay attention to the next steps
You must use a .png file for your wordmark. It must have dimensions of 250 x 65 pixels (or less). If your image doesn't fit, use a dimension converter. If your image is not .png use an image converter to .png. You can also change the size on that converter as well.
After changing the image, navigate back to your Theme Designer, and under the "Wordmark" subheading, hit "Choose File". Find your file (the one you converted) and hit "Upload".
___Troubleshooting___
After uploading your wordmark, it may say "The image exceeds the dimensions' limits". This means that the image is too big. Shrink it down with a dimension converter.
Hope this helps!