First you must select the appropriate scope in the Business Configuration work center, click Finish and then complete the milestone in the activity list called "Confirm Milestone: Design Accepted". Once the milestone confirmation is complete, you can log off and log back on to see the Business Partner work center. Then you have to set-up a service agent user and assign the work centers that you are looking for. If you need help, the Administrator Guide at http://help.sap.com/cloud4customer?current=on-demand provides instructions.
To gain hands on experience with the implementation process, I would suggest that you take one of the Implementation Experiences that we offer. You'll receive an enablement system and guides that will walk you through all of the important implementation steps. The links for the two enablement services are below.
https://www.regonline.com/SAPSalesODImpExp
https://www.regonline.com/SAPServSocImpExp
Have fun!