A freelancer lists all of the services they can provide to clients in the "Skills" box on their profile page. The more skills a freelancer lists, the more likely it is that a client will find their, so a freelancer should list every skill that they can offer.
Here's how to add skills to your profile:
Navigate to your profile page by clicking your name in the top-right corner and clicking the "My Profile" link that appears in the drop-down menu.
Find the "Skills" box on the left half of the page below the "About Me" box.
Click the blue "edit" icon in the top-right corner of the skills box, (the one that looks like a pencil). The "Skills" pop-up will open.
Select your skill categories in the top field. These are broad types of work that freelancers do. If you were to think of the types of services you want to offer on ZMZ, which of the categories listed would they fall into? Select all that apply.
In the second field, list your skills. Include any and all skills that you can offer.
If you want to display skills that are not listed in the ZMZ skill list - see this article for details on what to do.
For more articles on getting started on ZMZ, see this collection!