This is a large question that could be a very large answer, but I'll try to boil it down to the basics for you.
Mixing a pop or hit song is all about balance, simplicity with simultaneous complexity, and groove. And then specifically the mastering process that's added on the end can add quite a bit of that "professional sound too".
Let's break it down:
- Balance - Think smooth. There are no elements in the mix that rub your ear like sandpaper. Instead, everything bobs and flows together like the waves in an ocean. In pop, the vocal takes mainstage, so be sure to have a fair amount of compression and high-boosted EQ. Some saturation can also make it shine.
- Simplicity with Simultaneous complexity - Listen closely to hit songs and you'll notice that there are one or two memorable elements that shine, but behind the scenes much quieter in the background are a wide variety of stimulating noises, transition effects, and textures. This creates a ton of depth!
- Groove - Mixing groove is all about EQ and not squashing the heck out of a luscious sound. This is a classic mistake for engineers who are just beginning. Imagine the human breath moving in an out to a tempo. This is how groove should feel. Make sure that each element of the track feels restrained by compression so it's smooth, but still dynamic enough to have musicality to it.
Finally, mastering is a large part of making a track sound professional. My mastering chain is basically a master bus compressor to glue everything together (I use CLA Mix-down), a Multi-Band compressor (to enhance balance, glue, and smoothness), some analogue emulation plugins (Abbey Road Vinyl, NLS Summing), and finally a mastering limiter (Ozone). :)