What to plan before starting development on a project? [closed]

Question

Say I've received the specifications for a project from a client, and now its time to start developing it. Normally, I just start with the first module (usually user registration) and then go from one module to the next. I only plan in my head just before I'm about to start on a module how its going to work, but there's no planning before that.

However, I think it would be better if I went over the specs and planned out how the system was going to work before I coded it, e.g what are the main components, how they're going to interact, etc. I'm just not sure exactly what I should plan.

To give a better idea of what I'm asking for, how should I-

a) Divide the project into components,

b) Plan their interactions, e.g should I do class diagrams, write unit tests, etc.?

Any ideas?