FM to extract data from Datasource

Hi,

Can any one of you suggest me an idea or let me know where to start and how to start ?

I want to write a ABAP code/ create a Function Modules which extracts data from a DATASOURCE be it any on condition(say date) and the output given should be filtered on some fields

Does a standard datasource be extracted using a custom FM?

Are there any standard FM to do this?