There's a bunch of info on this site (forzamotorsport.net) about tuning.
When I tune a car I usually do the following. BTW, I'm not a great tuner! So if something I say here is incorrect... It is no shock to me!
1. Pick a car, and a balanced track (a quasi-equal amount of similar rights/lefts)
2. Drive the car around that track like 10 times. You want your difference from lap to lap to be tenths of a second
3. Then, start making your adjustments.
As to what you can adjust and why... here's an old Forza tuning guide on IGN. If you don't know much about these terms you can search them down further.http://faqs.ign.com/articles/613/613408p1.html
The big things to pay attention to are:
1. Tires, what PSI are they at after a few laps? Where are they hot, where are they not? Use this info to change your camber, toe, caster, and tire pressure
2. Then, I usually adjust downforce to help me keep speed in the corners. If a track is long and fast... you may want to lessen downforce. After doing this... give your tires another quick look as they may need further adjustments.
3. Then, I look at my suspension through the corners. If the meters are going red under pressure I stiffen my suspension until it very rarely goes red. you can make adjustments to this and anti-roll bar to find a good balance of body roll + suspension responsiveness. If you tighten up the rear anti-roll bar it will give the front end some more play + vice versa. Also suspension can help you set Brake Bias. If the front suspension is hit hard under braking you'll want to even that out by moving the bias to the front brakes more.
4. I usually lower my brake pressure a little bit on most cars to give me more control to hit apexes and not scrub too much speed accidentally.
Those are the big ones for me, and they help me get a car that is consistent.
Then I adjust bump damping and gear ratios. Gears are tricky, but tiny adjustments can make a big difference.
Watch what gears you are in and where. You might find that tightening 3rd gear helps you get out of 4 different corners quicker... little adjustments that suite your driving style are the way to go.
...And sometimes you'll find that an adjustment does exactly the opposite of what you had hoped, but you can take that knowledge and use it for good.