I'm new with OOP and UML and I have some confusion here. I'd like to know where to start, I mean, somebody comes to you and ask you to do something (involves software design of course), once you have determined what has to be done, what is the order in which you have to start the software architecture? I mean, is it first the use case diagram or the class diagram? or should I draw some diagrams in parallel?

But basically, where should I start? and what UML diagram goes first? Thanks for helping!!