On any world with an atmosphere, it's possible to use the atmosphere to slow yourself down and thereby use less fuel. The more parts on your ship, the greater it will slow you down. [This is not how actual aerobraking works. Right now it's based on part count, but in the future KSP will probably migrate into a ray-tracing method, where place the largest amount of surface area into your prograde will increase drag.]What
is the most efficient way to land?
Some say that it's best to just go straight for a collision course, and then slow yourslef down, while others would say getting into a proper orbit first is the best way. Both are wrong. The most efficient way is to give your self a periapsis that is as low as possible. (Around 10km is best, but on worlds with atmosphere, you'll have to take aerobraking into effect when planning yout height.) At your periapsis, face directly perpendicular from the surface, and point your engines towards prograde. Burn off all of your Horizontal Speed so your projected trajectory in the map view looks like a straight down line. Now, since you burned at periaps, you should have little to no velocity. Now simply Control your descent and you should have very little problems landing.Retroburns
On worlds with no atmosphere, the only way of reliably controlling your descent is burning retrograde. To do this, simply point your engines in the direction your falling and burn. If you've gotten yourself into a trajectory that is moving horizontally, look at your navball and you should see your prograde marker be off from the direction pointing directly down. To fix this, point your engines so that they'll be on the other side of the prograde mark from down on the Navball. Burning in that direction should slow your descent at the same time as fixing your horizontal movement. If this movement is more pronounced, and you're relatively high up, you can face perpendicular to the surface and burn, like when you first set up your fall.Parachutes
Obviously, parachutes don't work on worlds that don't have an atmosphere
. Keep this in mind.
Now, parachutes are the most simple form of descent. Simply slap on a few and pop them once you're in atmosphere and you're good. Parachutes work by creating as much drag as possible. You can view their force in the VAB parts tray. The yellow droge chutes do not provide as much force as the others, and the big blue one produces the same amount as the small white and radial parachutes, so it doesn't matter which of the three your using, just use the droges on planets with lighter atmospheres, or where you think you'll need to start slowing down earlier. Remember to use more for larger ships, so you have enough force.Landing Sites
Lastly, pick your landing sight carefully. You're going to want as flat and smooth a surface as possible. Try swinging the camera around in the map view a little to find a good place. If you're intending to land on liquid, make sure your center of mass is very low, so that you don't tip over.