Depends on your preferences, the trail you are riding, and what adjustments your setup has.
As a rule of thumb, you want your suspension to help you track loose trails, absorb big hits, and leave you with control rather than acting like a pogo stick and spitting you off into the weeds.
All the Rockshox forks I have are adjustable in terms of spring pressure, spring rate, rebound and compression damping. Spring pressure is the easiest to describe and adjust. If you have an air fork, you can determine how stiff or soft the fork is by adjusting the amount of air in it int he first place. Often referred to as setting the sag, you are advised to dress in full riding gear and stand up on the bike in the attack position. The fork should sink somewhere between 25 and 33 percent into the lowers. On a smooth fast surface with few bumps or drops I would aim for less sag. If you are going to ride bumpier more technical trails I would go for nearer 35%. That way you have a stiffer spring with less travel for smoother faster surfaces, and more suspension travel for bumpier stuff.
Rebound damping affects the speed at which the fork recovers its stroke after compressing. Too fast and the fork bounces you back up, thus reducing tyre contact with the ground. Too slow and the fork fails to recover after a hit and packs down, or simply doesn't get ready for the next bump in time. Yu should aim to adjust rebound to be as fast as possible without bouncing. It should compress and then smoothly reset. I test this by riding off a kerbstone and watching the recovery speed. You can also get a feel for it by holding the bike on the back brake and putting your body weight on the bars, then letting it back up. The fork should recover marginally slower than the speed you release the bars at, if you see what I mean. If the tyre lifts off the ground you have it set too fast.
Compression damping affects the force required to start the compression stroke. High damping means the fork needs more of a hit to make the fork work, and open damping means it's plusher to start with. Many forks have a lockout and compression dial on the right hand leg, which means it takes a lot of energy to start the stroke, and Rockshox forks have a motion control damper which you close right off to stop the fork compressing at all unless you hit a big enough bump to activate the blowout gate. Yu can then steplessly open the damping until the fork is completely active. Again, this depends on terrain, you can adjust the fork so it reacts to lots of little hits on a smooth chattery trail, or just just take big hits and work all the way through its travel on bigger stuff. Compression damping is useful to prevent fork dive under braking. And to prevent the fork plunging into its travel, steepening the head angle as you point it downhill. Trial and error to see what suits you and your riding on this one.
As for the rear, well, your initial aim is to balance front and back in terms of sag and rebound. Experiment with rear shock sag to find an initial spring pressure that uses most of the travel only on a big hit, rather than blowing through all of its travel the first time you hit a drop. I set mine to about 25% sag, but then I'm a Clydesdale class rider. To set sag, you need to be in full riding gear and simply sit on the seat. Adjust rebound u til it feels similar to the front when you bounce the bike on a flat surface using your body weight. It the rear rebounds faster than the front you'll end up over the bars. If the rear is slower the bike will squat in certain pedalling situations.
Compression damping again will depend on riding style and preference, and will depend on settings available. Specialised brain shocks do most of the thinking for you, and my Fox RP23 has three position propedal options and open position. I tend to leave mine in open position as the IBIS has DW linkage.
Loco suspension tuning has a useful setup guide, as does TF tuning, so have a look at their sites. They also offer to tune your shock to match your fork based on your weight and riding style.
