hello
for my project i begin to do uses cases diagram, the main goal of this diagram is to consider the final software like a black box. with this diagram you could show to non informaticien the final goal of the software. after that, i can advise you to do activities diagrams in fact, thees diagram can allow you to determine the futures interfaces.
i stop here explanations but if you want more informations
send me e-mail

good luck

Cédric