I'm not saying you all don't know the answer, I'm just saying that you're all wrong and you're all right. The problem is the question. There's no information except the levels before clipping and the volume on his car CD player. This person is not ready for more info just now, so the guy who suggested reading and reading some more is the most right. That being said, ears are the final judge of all mixing and mastering. A monkey can smear paint but that doesn't make him an artist. Anyone can learn how to get some tracks and put them together but that doesn't make them a recording engineer. This person right now just needs more volume. Most programs like cubase have a mastering multiband compressor with presets. He should run his mix through that and A/B them to see which one give him what he wants for now. Mastering his mix won't hurt it if he saves a copy first, then masters one of them. He needs to practice doing. Then he can learn what he wants. Imagine him trying to work with a mastering engineer when he has no idea what he wants? (I can tell because he uses the word "louder") What he means is "more like finished CD's that he buys.". To do all this right he needs to learn these things and the differences between them.
1- Gainstaging
2- Mixing (starting with the bass and vocal first, then drums, then other instruments.)
3- EQing (Using a parametric; cutting out the bad as opposed to boosting the good)
4- creating the mix as a separate file.
5- Copying the mix
6- Using the multiband compressor with A/B tests.
This will help most amateurs get what they are doing right. Most amateurs have never been near a real recording studio and have never read anything in the recording magazines or bought a book on recording. One thing is always true. Books are cheaper than the recording equipment and far more valuable. I wish a lot of musicians would go buy the books first; then you wouldn't see so many questions here about why this won't work with that.
I copied all of the fundamental articles from all of my recording magazines; things like frequencies and gainstaging; things that never change, and put them into three large binders. I also asked questions every time I worked in a studio. I built the first home recording studio in NYC and was recording live with MOTU Digital Performer while their program still used line drawings for mixing boards. It took me three years before I had something to play for an engineer. He was amazed I had managed to get that level of quality in the recording. I told him I stopped trying so hard. I let the mustic do the work and just tried to capture things accurately. So, try each of the things suggested here but do it on copies of your mix. See what works for you. Learn as much as you can, both by doing and by reading and trying it out. Good Luck and welcome to the world of recording.