I believe it is quite simple, but most newbs can't see it. Newbs usually run out, buy big subs, and overlook the fundamentals.
Before you waste one dollar on equipment, you need to find out what your goal is for your vehicle. What do you REALLY want out from your stereo?
You need to figure that out, then work backwards creating the events and performing the steps necessary to obtain that goal.
If your goal is to be very loud, you want to start from the Alternator outwards. Every system I do from now on will be in this exact order:
First, maximize your alternator system. Second, upgrade your big 3 to 0 gauge. Third, upgrade your battery. Fourth, upgrade your fusing and (+) run to the trunk with 0 gauge . Fifth, add a trunk battery. Sixth, start figuring out what dampening will be needed to make your car sound best. Seventh, start figuring out what head unit you want. Eighth, start figuring out which front stage matches your cars acoustical environment best.
And then after all that is done, figure out your sub stage.
It seems like a lot, but believe it or not, this is exactly WHAT you should do if you do not plan on upgrading every other month and want to preserve your money.