SAP BI has many areas to master. You need to decide if you want to focus on the front-end or back-end.
For front-end, you will specialize in Business Objects Business Intelligence and learn the reporting toolset. You will need to develop skills for gathering reporting requirements, and visualizing data graphically or in tabular format.
For back-end, you will specialize in designing data warehouses in SAP BW or SAP HANA. You will need to develop skills for data modeling and ABAP programming.
For certification, go to the SAP Education site and look at the recommended courses. I also recommend the SAP PRESS certification guides (eg. BI 7.0) to help study for the exam. You can also find lots of training videos and free content online thru SAP.