Regarding loudness
For most modern styles of music, including pop, electronic, rock, rap, etc — anything with beats or a drummer, basically — I generally try to bring in the master no quieter than -8.5 LUFS integrated.
(Diyers who are making recordings at home and may not have a formalized mastering process: you can read “final version of the song that people will hear” in place of “master.”)
This is competitively loud. So, why do we want our masters to be loud? Because we don’t want our recordings to sound weak when people hear them in context of other recordings in their genre. Loudness shouldn’t matter, but it very much does — all things being equal, people think louder things sound better.
"LUFS integrated," if you're not familiar, is a standard for measuring loudness over the length of a piece of audio. LUFS stands for Loudness Units Full Scale; "full scale" means that it's the kind of measurement where 0 is at the top and you go down to -∞ as things get quieter. And what is a Loudness Unit? This is oversimplifying, but it's basically a decibel.
If you don't have a metering plugin that measures LUFS, here's a good free one.
Here’s how to measure the loudness of your song:
Put your LUFS measurement plugin on your master bus — make sure to put it after your limiter, so it takes the effect of the limiter into account.
Set it to measure LUFS integrated
Click its reset button to zero out its meter
Play your song all the way through from the beginning to the end.
When the song is done playing, you'll know how loud it is! And that's valuable information.
Loudly — jamie