This is a very broad question, from a general "where to start" to very specific about Open Source. I'll focus on the "where to start"…
Regarding the size of this project (ERP) I would start with figuring out if you have a market for it. You say you have no real client as yet, but do you know what you need to convince potential customers to implementing your solution? Can they start with a single module or do you need multiple modules available before you can actually publish something?
A project needs a clear goal, otherwise it will start drifting even before you have started .
Next, do you have a team available? Even though it will be open source, will it be build by a commercial team or by volunteers around the globe? Maybe the project should start with some marketing to get volunteers enthousiastic. This kind of project should not be initiated by a single person alone.
Regarding a project Lifecycle depends a lot upon the approach you have chosen or will choose to develop this thing. For now, start with three phases: Preparation (or initiation, what you're doing now), execution and closing. The execution phase can later be subdivided into technical phases (e.g. by release).
Now to the organisation of the project itself (sounds more like a program to me), the first thing to do is to decide upon the scope of your project and communicating this to all your stakeholders. A Project Initiation Document helps in this respect, but personally I start with building a deliverable-based Work Breakdown Structure (WBS) to discuss and decide upon the scope and supporting processes (QA, training, marketing, …).
The WBS will be the basis for that, as well as for creating the PID, some kind of schedule, etc. Since you have your "Project requirements" available, creating a first draft of the WBS will be relatively easy. Furthermore, some kind of 'budget' is necessary, either in money or in number of hours if you're working exclusively with volunteers, to set expectations straight.
Note that 'documents' are also deliverables on your project, so if you want to create a user guide, then it should be visible on the WBS as well. Make a decision about any documentation you think is necessary.
If your team will only meet 'online', than it is a good idea to start with a collaboration environment. However, I would start with a shared dropbox folder or whatever (just keep it simple) untill you really have the project started.
Some final points to complete the PID:
- Use your WBS as input for an initial risk analysis.
- Finally, determine how to track and communicate progress. Again, the WBS can help here.
When this is clear and agreed to, you can start.