Statspack. Great place to start.

May I suggest you also get Tom Kytes' "Effective Oracle by Design". It'll really help you understand what the numbers mean.

Chapter 4 is probably where you'll want to focus.

Here's the table of contents.
1: The Right Approach to Building Applications
2: Your Performance Toolkit
3: Architectural Decisions
4: Effective Administration
5: Statement Processing
6: Getting the Most Out of the Cost-Based Optimizer
7: Effective Schema Design
8: Effective SQL
9: Effective PL/SQL Programming
10: So, You Had an Accident
Appendix A: Setting Up and Some Scripts