|Think about Loose Coupling|
Reading between the lines of your question, I'm gathering that you are going to be chief-cook and bottle-washer on the project. I find that is is just as important to be disciplined on a one-man project, but n-times more difficult. Documentation, clean code and rigorous testing against requirements are just as important on a one-man project as a 100 man project.Even if you are the customer of this project, follow the process. You'll thank yourself later.
Leaving aside the excellent project lifecycle comments already posted, I would make a couple of suggestions on the design and coding part.
First, there are 2 books that you should read before tackling a large project. They both by Microsoft Press (stop laughing! No, really I mean it!) Code Complete and Writing Solid Code, both are excellent studies of how to code effectively, and how to test effectively and how to debug effectively. Conclusions as to why Microsoft doesn't produce better software aside, these are indispensible resources.
Second, if you are looking for a place to begin- think data and data structures. As James Brooks says, paraphrased from The Mythical Man-Month, (which I don't have in front of me) "Show me you code and you've shown me nothing, show me your data and you've shown me everything". (Actually, make that 3 indispensable books. Read Brooks as well, especially his "plan to build two, you will" essay). Anyway, think about you data. How it will be store, retrieved, manipulated, past around, verified and tested. Build tables, normalize, set boundry conditions, determine dependancies and relationships. Once you know everything about your data the code will write iself.
Third, plan and built test cases from the very beginning. As you add features add test case to test them. It will be much more effective than going back later and trying shoehorn tests in. Look at the testing tools that come with perl, or search PM.
Keep a todo list and a changelog. Each day update the changelog with bullet items of what you added or fixed. It will motivate you and give you a sense of accomplishment. Everytime you have an idea for a feature add it to the todo list and periodically scan it. (Neither of these are substitutes for a good MR systema nd a good source code control system).
Without knowing more etails about the project I can't give more advice.
Good Luck and Think Big,
Entropy is not what is used to be.