SQL & PL/SQL

I have recently written my first stored procedure.
Its rather a bunch of SQL statements.

I had to hard code lot of dates. most of them are first day of the current monthe or last day of current month etc.

I thot of parametrizing all the dates, but if a business person has to include all the parameters they could go wrong and get the wrong results.

Now, I want to use date functions to achieve these requirements:
Can any one please throw some insght into this:....

1) First day of current month,
2) last day of current month.
3) first day of previious month
4) last day of previous month
5) first day of a month that is 3 months from now.
6) last day of a month that is 3 months from now.
7).....

Can any one please throw some light on any one of this.. I can try to work from there onwards ....

Thanks a lot in advance,
Ac