How you do it depends on the design of your system as there is more than one method possible.
The basic method is to use a script that regularly checks for records with a date within a specified number of days of today's date and pops up a message and lists the records if any such are found.
How you set this up for regular checks can differ. The most basic, is to use File Options to specify that this script run every time you open the file or the first time you open the file each day...
great thanks, is it possible to link 2 databases together, as an example contact management and task management
Yes, but for complete solutions such as the starter solutions of that name, it's not a trivial process. If you build your own solutions from scratch it will be much easier to produce a combined system that manages both contacts and tasks. With the starter soluions, both have tables designed to store the same data, to link them and not have to store the same data in both tables, you'd have to replace references to a table in one file with references to the similar table in the other. Since fields and their options aren't identical, this can require a lot of effort tracking down each reference to each field in the two tables in order to determine how and where they are used, then "repoint" these references from the table you plan to remove to refer to a field in the table you plan to keep...
Needless to say, this all is much easier to do if you have a good working knowledge of FileMaker and relational databse design. You might investigate the "business productivity pack" that FileMaker offers. I think it has a system similar to the starter solutions, but merged into common tables so this work would already be done for you.