indexes, sorted table, subqueries and other performance tips.

Hello experts,

I was just given a report by my boss that needs to be optimized. As I browsed through the report, I noticed that some of the comparisons in the select statements uses fields that are not primary keys. So my teammate said that I put indexes and instead of sorting the internal table after all of the select statements, I wonder if a sorted table is much better. Also, I am curious if using subqueries is better and faster that say, FOR ALL ENTRIES. I am currently new to ABAP so if you guys can post examples on how to put an index, how to make my standard itab to a sorted table, using subqueries instead of for all entries, etc. Thanks a lot guys and take care!