I'd suggest getting a 4 channel amplifier, doesn't have to be super expensive. Get some decent coax speakers up front and bridge the rear channels (wire the + of one channel with the - of the other channel) for a subwoofer.
Something like this for the amp: http://www.crutchfield.com/App/Produ...9AP1040&tp=115
It is an inexpensive amp, it isn't super flash and it isn't super powerful, but it is better than the 15w your current H/U is putting out.
As for some coaxial speakers (those thare the ones that have a 6 1/2" cone with a tweeter or two attached in one unit) look for something in the $50 range. Should be an improvement over what you have currently. I suggest shopping at a place like crutchfield.com especially for beginners because they will include everything you need in your order and tell you what will fit your vehicle. They also have great technical support during and after your install.
As for the subwoofer. I'd look for a 10" sub and you can get a box from Tacotunes that will fit behind your rear seat. The image dynamics sub that they reccommend is a fantastic subwoofer. It is has a nice blend of sound quality as well as having the ability to get pretty loud.
If you are a more a/v savy person, there are better deals to be found but rarely will you find the customer service of crutchfield.