basic computer applications

hello
i am student of Bsc(hons) Management and i am doing a module Basic Computer Applications..
i am doing an assignment on a shop that sells plants medecines for plants and gardening tool..
i must show that they use database at work.. i am really very confused i must submit my assignment next tuesday.. please can someone help me cause i have done research everywhere but in vain i would like to know how to do an ER diagram… how do we do it?? and how do we construct a use case.. ia m searching how many staff do this type of store employ in including the manager and how what is the management hierarchy??pfff i am afraid i would be able to complete my assignment… plizz help me
can someone plizz help me
i would be very grateful