Hi:
Process modeling leads data modeling, so start there first.
A business system consists of software and/or computers. Do not fall into the trap of thinking that to capture the behavior of automated processes requires a different technique than to capture the behavior of manual processes. That is a forced, artificial partitioning, the main damage caused by such is that it retards integration efforts - especially on larger scale efforts. Only one technique is needed.
Do not use process modeling techniques that try to steer you into prematurely thinking about solution. Thinking solution too early is a sure way to derail essential requirements analysis. In terms of BUSINESS analysis, its a data flow, not a message, its a process, not an object. Leave thoughts of messages and objects to development staff. Remeber, one of the most difficult parts of requirements analysis is to maintain a proper focus. It has always been my experienced that BA's who are not strongly focused on the essentials largely miss them.
Start top down as much as possible, else, there is a real tendancy to "drown in an ocean of details". But be aware: business systems at higher levels of abstraction (i.e., at the bigger picture level) are notorious for being non-sequential. One can not model the non-sequential using a sequence dependent technique. Many, including some of the "experts", dispute this very imporportant point saying, in effect, you can, even at the top most levels, identify a sequence (including a start point). Try to do such yourself. If you can not, then try data flow diagrams (the only real non-sequential technique suitable for larger scale manual/automated systems behavior documentation).
In conjunction with top down analysis, a critical task is to model scope. A context diagram is the way to go.
When you are modeling processes, think of the essentials. A process is defined by it inpts and outputs, which are typically data flows.
For identified data flows, start a data dictionary. Not the techie type, but, simply, plain english definitions. This is the primary input to your subsequent data modeling efforts.
Above all, do not let the techie types steer you into thinking that requirements analysis requires understanding a bunch of techie stuff, including techie stuff related to how you will proceed in requirements analysis. For a truely Agile approach, all you really need is a few higher level business oriented data flow diagrams, some entity relationship diagrams, and maybe some screen shots. You can leave all the detailed requirements and the techie stuff to be handled by the developers in coverstations.
Tony