In your list of jobs, clients, suppliers, quotes, expenses and invoices, you'll see that there is a star in front of each item.
You can click on this star (and then refresh the page) to pin the item at the top of your list. The star then turns yellow.
To unpin this item, click on the star again (and refresh).
Note that when you pin a client at the top of your list, this client will appear first in all dropdown menus in the application, with a * next to it (e.g. when you select the client when creating a new job).