Starting out to build a full car amp with SMPS for a first step into electronics is a huge undertaking. What I'd recommend is to buy a cheap four-channel car amp to use for your speakers and sub. Once you have it, do a lot of research on this, and similar, forums and google for schematics, concepts, and topologies of power amps and DC-DC SMPS circuits (DC-DC means a DC to DC converter, or a circuit that takes the 12V DC from a car battery and generates the + and - DC supply voltages (suck as +/-35VDC) for the amplifier circuits) (SMPS means Switch-Mode Power Supply). Then you can refer to the amp your purchased, reverse engineer it with the knowledge you gained. Once you're comfortable with the various circuits that make up a car amp, then you could go about gathering the parts to build your own. In the mean time, you'll have the store (or eBay or whatever) bought amp to use to listen to and learn from.