Guys and girls, please keep it calm, there's no need to attack each other.
As a start, I can offer the
Aero Glossary, which should help understanding some of the basic stuff that teams are using.
As @ESPImperium mentioned, there's never specific area to start, but most of the time you carry your design philosophy from previous years, unless you want to break up with the past, just like Ferrari did a while ago, introducing pull-rod at both front and back of the car. They [Ferrari] have also chosen to go for very small, yet apparently useful for them, upgrades, which usually consist of small vortex generators and turning vanes, slight changes in diffuser's gurney flaps, etc - looks like they are convinced that the core of the car is OK. Recently they have been trying new front wing approach, which is generally a big step, especially for them.
Mclaren is team which has picked up the pace recently - they are quite brave with introducing aerodynamic parts, take as example the turning vanes over the sidepods shown and raced at Spa. Previously, Mclaren had two or three (different throughout the races) vertical vortex generators on top of the sidepod, but now we have those new elements (a la Sauber), aimed to keep the airflow attached to the sidepods and create better exhaust flow management.
The team has also introduced those small winglets around the mirrors, much a la Ferrari, again, with flow conditioning in mind.
Note: Don't think for a second that I imply Mclaren are copy machines - not at all. This happens all the time in F1. The hard part is to actually make all those tiny changes to work together.
So, where do you start with aero? Once you have a solid core concept, you can start adding aero elements. This usually happens when the design office shapes the car around the next year rules.
I can talk for ages about Red Bull and Lotus which have very good CFD departments, but that's another, quite exhaustive topic and I shall write about it later, when we have bigger pauses.
