Personally I've tried a couple of times, but never really persisted. The main problem is that I don't really know what goals to set and what to look for. What settings do I work on first and what must I try to achieve with them? It's completely beyond me... I'd be interested to know how you tackle that.
My goals are always changing, depending on the car, track, and how familiar I am with the combination. Assuming both are totally new, my goals start at finishing on the lead lap with 0 inc, and move up to finishing within top 10, top 5, podium, etc. In order to do that, I need the car to fit my style, which is one that starts loose, works it's way toward neutral, and never gets tight. Since the majority of supplied and shared sets are far tighter than I can manage, this is where things get interesting (or down right infuriating).
I'd start with eduacting yourself in the basics of what adjustments do. In addition to what Niel provided, the iRacing forum is full of information, ranging from handy, 1 page quick reference charts, all the way to multipage breakdowns of each and every adjustment and exactly what they do. Don't try to absorb it all at once, just find something easy to remember and understand and start there. Trackbar is my favorite. Up is loose, down is tight, and that's about it. Of all the adjustments, this is probably the most direct since it doesn't really affect much else on the car.
Once your comfortable with that, it's on to shocks and springs, camber, and tire pressures. Here's were the difficulty begins to rise exponentially. Any change you make is going to change something else on the car, and often changes cause multiple reactions. Say you want your car to be looser in every part of the corner.You stiffen the RR spring and find you're loose through and exiting corner, but you're still garbage entering. So you throw on a stiffer LF shock to loosen corner entry, but that will in turn tighten the middle and exit and negate the affect of the RR adjustment. In addition to the counteracting effects, these changes change nearly everything else about the car. With stiffer or softer suspension, your tires will travel differently, requiring camber adjustments. Changing camber changes the ride height, requiring height adjustments. Changing ride height changes the weight transfer, requiring shock and spring adjustments. Shock and spring changes affect the travel, requiring camber adjustments. See where I'm going with this? =D You can drive yourself not only crazy, but away from the sim entirely with just how frustrating this stuff can be, and this doesn't even include caster, stagger, ARB preload, and all that mess. I don't mean to ramble, but rather wanted to give an idea of what's in store rather than just saying "it's hard" and leaving you clueless.
My suggestion, in addition to a little reading to familiarize yourself with the adjustments, is to begin with one of the lower class cars. The changes available are few and will give you opportunity to tweak and learn without becoming overwhelmed. What I do is hop in, do 15 laps or so to feel the car out and bring my tires up to temp, then hop in the garage. Tire temps will tell me what my car is doing. Temps are read as Outside-Middle-Inside (OMI) or IMO depending on which side of the car I'm on, and they should be close to similar for each individual tire. A tire that has a much higher middle temperature is over inflated, a much lower middle is under inflated. A high outside or inside temp relates to camber; too much or too little depends on which side of the car your looking at. Lower class cars will allow you to get familiar with changing these values without worrying about ride heights, ARB preload, shock collar offset, and all that mess. Tire temps should also give clues to how my car is handling. Although the RF should be the hottest tire of all, a RF that's 30-40 degrees hotter points toward a tight condition as I'm overworking that tire. A RR that's within 5-10 of the RF shows a loose condition. Knowing how to read and digest these numbers can point you to where you need to adjust. Even if my car FEELS terrific, a RF that's 40 degrees hotter is going to burn up too fast and is something that needs to be fixed, regardless of how it feels.
I wish I could help even more, but to be honest, I'm just scratching the surface myself with getting into the more intense adjustments. I spent 6 hours reading and 2 hours testing Trucks last night with limited success. I managed to peel .6 off my time, got within a second of pole and moved towards the balance I needed, only to try to fine tune it more, knocking it out of inspection, "fixing" it again, and ruining it entirely lol. For now, I try to work first with tire pressures and camber to get my temps right, then start messing with those, the track bar, and mild shock/spring adjustments to get the balance. I go back and forth between doing these two objectives (temps vs balance), and then I just run it. At this stage of my game, too many adjustments gets me in a heap of trouble. Hopefully in the coming weeks, I'll be able to put together some stuff for the next season and supply some solid tips in the forum.