You need to ask yourself some questions about the project first to better understand what and how will be realized.
Here are some key points you should consider. These are only suggestions, or points you may want to take into consideration or not. Everyone has a different way of doing things, just think about it.
By NO WAY this is a complete list!
1) Gather info
Ask as much details about the project as possible. Both to your contractors, and to the people who will actually use it. Try to understand what are the expectations about this application and what it will be used for.
2) Understand what the project should be able to do when finished
You may want a detailed analysis about your project before actually thinking how to build it.
What are the functionalities it must have?
What kind of commands and data the users expect to see and use?
How should the application being structured?
You can make sketchs using paper and pen, post-it, or using some software to help you keeping tracks of your ideas and thinking.
3) Choose the right tools for the kind of application you need to make.
There are many programming languages, databases, server typologies, frameworks, etc. out there. You have to choose the right tools for the job, based on what you actually need to develop, and what you feel most comfortable with.
Sometimes you cannot choose any of this, but if you can, choose them well.
4) Start making some prototypes of the user interface.
Again, you can use paper or software or both or what you think it's the most useful tool. Just start to have an idea of how the application will look like, where to put the functionalities you need, the commands, where to display the data, etc.
This will give you a better idea if what you thought before is ok or if you need to change something (back to point 2).
5) Start to prototype some code functions.
You may want to be sure you are actually able to do what you are going to do before you are already halfway on your coding and understand (too late) that you went the wrong way.
Start coding the key-functionalities of your application and test them to see if they work as expected.
Perhaps the programming language you choose is not so well-suited for the kind of task?
Or your skill with that particular language/framework are not so good and you feel you could do it better or faster using other tools?
6) Show your contractors your prototype, both the UI and the basic functionalities.
You only prototyped a basic UI skeleton and application functionalities so far. But it's already enough for other people to see if this is what they expect or not.
Gather as much feedback as possible, understand what's wrong and what's work ok. If something is not working as expected you will have to rethink about it and change your project accordingly.
7) Start coding the application in detail.
Now you can start implementing all the code and UI details.
The order in which you develop the application is up to you, and varies depending on the application.
Just focus on one small piece of the application at a time, divide the project into small pieces.
I won't go more far than this since the question was only about starting the project, but I think you have some good points to think about.