Add a field?

Add a field?

I want to add a new field URL to my contact page. 
When I try I get this error. 
Maximum field limit reached.

Only 15  custom fields can be created for your edition.
Please  upgrade  to Higher Editions, or remove the extra fields and delete them permanently  from  Unused Fields  to create new fields.

How do I know which are the custom fields so I can remove one and add the URL field.
Thank you. 
John Gurba
courtaccess@gmail.com