Drill Down Reports

Hi All,
I need ASAP the following info if somebody can give me good guidance and help on this soon that is really helpful.

First Question
>I have a Main report with around 7 user parameters I have to generate other 6 reports based on the same parameters.
>I have to I guess go to drill down reports and I have not done any so can somebody help me asap with this in detail since I have not done it.

Second Question:
Second thing in my main report I have a total count of people and is coming out of a from summary column.I need to get percentile value for this ie say the
(Total count) * 0.25 = Say 250 (Which is nothing but the count) so at 250th Count I have to find out the score at that count.Can somebody help me on this too.Your reply is very much appreciated.
Thanks