I want to paint the trim around my radio/ac and also inside of dash around the gauges.
1. what paint's should I use?
2. how should I prep it ?
3. should I clear coat ?
Krylon, Rustoleum, and others all now make paints designed to adhere to plastic. If you can't find them in a color you like, you can use Duplicolor Adhesion Promoter on the plastic parts in question, and then paint them with pretty much anything you'd like.
As far as prep, if you're dealing with parts that just need a light cleaning, use either an aerosol glass cleaner or some rubbing alcohol and a microfiber towel, then let them air dry. If you've got bigger problems, you can see my thread here: Progress Thread - Interior Panel Rehab | Mustang Forums at StangNet and see how I cleaned 40 years worth of dirt and gunk off some interior panels for my car.
Clear coat is completely up to you, but it will also depend on the paint you use.