composite index vs normal index
Your question amounts to, "How do I use indexes?" You should read some Oracle documentation to get up to speed on how to use indexes and how to determine what columns to index. Oracle has an entire library of free online books and documentation for their products. A few good links to start:


The chapter on Indexes and Index-organized Tables from Oracle Database Concepts (the entire book is definitely worth reading)
The Oracle Database Performance Tuning Guide on Choosing Columns and Expressions to Index
Using Indexes in Database Applications from the 
Oracle Database Advanced Application Developer's Guide