Re: Sound level question...
the songs you purchase from iTunes etc have all been mixed and mastered professionally.
Logic has some plug-ins to help you get a your tracks louder - more like what you'll hear on CD's and the radio, but it can take some time to learn how to do this.
Many pro's use 3rd party plug-ins by iZotope and waves etc to master their mixes for commercial release.
With Logic's built-in plug-ins I'd take a look at the compressor to see if you can add mre punch and use the adlimiter to give it more loudness.
If you don't already, think of the mixing and mastering of your song as two different processes. Do your mix first until you are happy leave a few decibels of headroom. Bounce it down (without normalise on).
Now master your bounced file to bring it's loudness up over the entire frequency range. Remember that many musicians give their finished mixes to pro engineers, not only because they have pro mastering equipment and studios but they have years of experience in creating masters.
One of the tricks in mastering is to create a finished song that will sound good on all systems (from your car hifi to a tiny radio to a prisinte Hi-fi)...
Hope this helps, and let us know if you need any more help.
Do check out the mixing tutorials for electronica and R&B as this might help you to create a better mix using EQ, Compression and reverb etc which will be easier to master.
Also check out this Wiki page on mastering: