for a beginning system, i would suggest you get a 4channel amp with built in crossovers, and get a 10 inch sub.

use the front two channels to power your stock front speakers with your crossover set at about 70 or 80 hz. they connect your sub to the bridged rear channels of your amp with your crosoover set to 80hz or 70 hz and down. this will be sufficient. when you have money again, replace your front speakers with separates. when you have more money, you can invest in a dedicated subwoofer amp and another 10 inch sub, para pair na.

dahan dahan lang. just make sure that the pieces you purchase can be integrated in your future system.