OMG what am I supposed to do?
Erm your job after all you are the IT guy with a diploma
(only pulling your leg).
Sounds like you have been thrown in a bit at the deep end and this is no bad thing, I'm sure your employers want to 'test' you out and it will give you a chance to learn very quickly.
First thing is don't panic take it one step at a time.
Excuse my ignorance but doesn't an IT diploma have lessons in (at least basic) networking? If so you should have some of the skills needed, even though you may not have any experience of networked accounting software that 'part' of the project is part of the application layer and essentially any decent software can be set up to work on a network without intimate knowledge of the function of the application.
Galaxoim has valid points even though this is not an Access specific project.
My advice:
In any project you must start by getting a clear specification of what the application is supposed to do. Otherwise you will be faced with "feature creep"
Absolutely vital it will also be very useful to find out how they do their accounting at the moment, what software they use, how it works, how they currently 'send their data' to well wherever they send it (if they do) if that means visiting the other sites then do so (you'll need to do this anyway at some point).
Find out what type of network they have in place at the moment. I assume each branch will have some sort of connection (internet at least), so check what type of connection, internet or company based network, who installed it and/or supports it (they are a good source of information).
Is their any budget at the moment (I doubt it by the sounds of things so you'll need to make a budget – don't worry too much about this for now though).
That would be a good start, now as I mentioned don't panic if this all sounds a bit daunting I highly doubt they expect you to do everything i.e. actually make the application, install and configure any additional hardware, maintain everything, train the staff, pay the bills
.
there is much more info I could type out but for now I would recommend you gather as much relevant information on how the 'system' works at the moment. Don't be afraid to ask questions, you boss is there to support you and if he/she is any good they will help you get started, give you contacts etc.
Also be honest even a small scale project can require multiple people with multiple skills especially when it come to IT and the information gathering process will give some idea of what will be required, what you can/ cannot do etc.
You probably find out that the easiest, most cost effective and best answer is to buy a 'solution' that will fit your current and potentially future needs and have it installed and set up by a specialist company. You'll essentially be the project manager
and yes this may not be what they expected but not only do you need to educate yourself you'll need to educate your higher ups.
Good luck