How to configure SSRS (SQL Server 2012) to run on a different instance, but same server as SharePoint
Are you sure of your version of Sharepoint? I thought SP2010 could not run on SQLS2012 but 2008 and 2008R2.

With which machine do you try to access the report server ? 

If you have SP and SSRS native mode installed on the same machine you will get error due to the address (SP and Report Server use the same). You can change the report server address in the Reporting services configuration manager (Start, Microsft SQL server 2012, Configuration Tools) and Report Manager URL tab.