Indexing issues, help

Hi, Can anyone clear me up about index, clustered index. 1. What is the best timing to use clustered index? 2. index A and index B has the same indexing columns but different order, such as (a, b, c) and (c, b, a), where a, b, c are columns. Will ithe proformance make difference that the table has these two index toghther? 3. What is the best indexing stradegy? Jack