Lots of people do side work for this stuff. Check the ricer hangouts (seriously) for those experienced in audio stuff. I had mine done at a place in Maryland that competes. They did an amazing job with the amp and capacitor.

If you have an amp, you will need a capacitor for it, otherwise the bass will sound like crap. Get one. Don't get cheap.

Installing speakers and a cd player is very easy. With the amp and capacitor, need to run the lines from the trunk to your battery.

If you want a custom enclosure for your sub, take a ride down West Broad St near Willow Lawn as there are a bunch of places there with varied experience -- ask for samples of their work. Good ones will have a portfolio of pictures of custom work and are proud to show them off.

If you just want a sub box, they are very easy to make. Use MDF board (Home Depot, Lowes). It's the super heavy/dense stuff that looks like cardboard. Use lots of wood glue and get the nice Simpson wood screws (in the Simpson Strong Ties building materials section of Home Depot and Lowes). Many install shops will sell you the box parts, carpet, and wiring you need.

Putting your own effort into an install puts a bit of pride into the work.