A newbie needing help

Well, first question is what kind of music do you listen to?

Second question is - are you looking to shake the pavement, bass that's overwhelming with respect to the music, or are you looking for a sound-quality system that puts the bass at loudness levels proportional to the recording?

This will help us decide what kind of money you'll need to throw into the subwoofer portion of this setup.

Next, I'd purchase a head unit that has crossovers built into it, which will let you filter the bass from the speakers. It's unbelievable almost how much louder you can turn up your speakers, when they aren't bottoming out on the bass... even on relatively low power.
Or alternatively, an amp to power your interior speakers, that has crossovers built in.

As far as shopping for speakers goes, just use your ears.
I've had the chance to audition a bunch of speakers lately... lots of good bang for the buck out there, particularly if you are shopping for coaxials, rather than components.
Components will tend to be able to get louder, if you can really power them up - mostly because of the crossover network that they come with! (see a pattern? It is really important)

Next would be the amp, if you didn't buy one already, for the main speakers.

What sort of a budget do you have?

 

well for $500 it will be tight.

First what type of Compents are in your car now, I.e Factory Stereo, Factory Speakers etc.

For the sake of this post I will assume everything is stock, which puts you in a Pickle.

you either

a> Spend you Budget on the Sound stage, getting a Head Unit, Amp and Full Range Speakers.

or

b> Forget about the above get a LOC and opt for getting bass only

there are draw backs to each. option A would not get you the bass I assume your after, but would get you better sound quailty

option b would get you the bass but would not get you very much in the way of soud quality as the factory speakers could not keep up.

Tell me which option you want I will mkae suggestions

 

I think the bottom line is, for $500, you probably won't be blasting everyone away.

You can pick up a couple reasonably inexpensive 12" subs, you'll want a ported enclosure, and therefore subs that'll work in ported enclosures well.

Since subs are more efficient in larger enclosures, you'll want to get the biggest enclosure you can fit in your car, which means you'll need to purchase a smaller amplifier...
...how much power your subs need to reach their maximum output level is much more a function of the enclosure, than the subs themselves, since the air trapped inside works like a spring - the smaller the enclosure, the more difficult that spring is to compress (and despite common intuition - ported enclosures actually face greater pressures inside than a sealed enclosure).

As far as "decent" speakers - it's all relative... relative to what you already have, stock, in the car... relative to your own personal experiences.
You can find a set of aftermarket speakers for your car probably for $30... will they be better?
...better than... what?