One of my colleagues Joy Beatty wrote a blog post on this topic here:

http://requirements.seilevel.com/blog/2011/02/a-business-analysts-checklist-for-launching-projects.html

We probably have a few other articles on how to kick off or launch projects. 

You might need to be more specific in your questions. I would say before the project starts, collect all the existing information you can about the system. Make sure you understand the value of the project. Identify who the key players are and establish good relationships with them. As far as how do you know when to start the project, I would say that is usually dictated by funding and availability of other team members.

 In addition I would include the need to Identify / request the project methodology being adopted on the project (i.e. if there is one)

The project method (e.g. PMI, Prince 2 etc) will indicate to you what you need and when to start and end a project as these methods break down projects in to manageable stages.

Here is my advice:

ID / adopt a project methodology

Capture the basic information which will enable you to easily start and manage the project e.g. who makes up the project, the various stakeholder who can affect and be affected by the project, what systems are being used by the stakeholders associated with the project, the business processes associated with the project as your project may cause the need to improve processes

Project budgets

Project document templates

Define a communication strategy which will enable you to communicate the progress and changes of the project as the project progresses.

You start the project once the business case has been signed off by the project board which is indicates you can go ahead with the project.

Develop your project plan (which is updated as the project progresses) and use it as a guide through to completion. 

I hope this helps?