The Account Owner, Admin(s) and Account Manager(s) for your account can create and edit Custom Fields. Select Settings (top right) and then Custom Fields & Templates. This opens a dialog box where you can create, edit and remove Custom Fields.
To create a new Custom Field, enter the name at the bottom where it says "field name" and select +new.
This opens a new dialog box where you select the type for the Custom Field -- a (1) currency field (in any currency you need), (2) drop-down, (3) checkbox, (4) text field or (5) number field. If you choose dropdown, you can also enter the options to present in the dropdown. Then select Save and that's it . . . you've added a Custom Field for your whole organization to use.
Note: Custom Fields and Templates are available to Standard, Professional and Enterprise subscribers.
Now that you've created Custom Fields you can organize them into Templates. Here's an article on how to create Templates.