Dock lines should be nylon, because stretch is wanted to relieve tugging strains, and it is usually single braided. For boats up to 30 ft, use ½-in line. From 30 to 40 ft, use 5/8-in line. From 40 to 70 ft, use ¾-in line. Avoid oversize line that might not stretch enough. Bow lines are usually about 2/3 of the boat length and spring lines are the full length of the boat. Dock lines tend to chafe through quickly and need some sort of chafe protection. A length of stout hose or tubing, larger than the line and drilled for twine at both ends, works as well as anything. Keep it in place by tying the twine securely to the dock line.
Well before approaching, lay out two dock lines forward, two aft, and two amidships. Have handy also a heaving line of double braid, strong enough to haul the boat but light enough to throw a good distance. Lines should be as long as possible with the understanding that they must not foul the prop and you can heave a line only about 20 feet. Lay out boat hooks forward and aft. Lay out fenders, but do not rig them overboard until you know for certain that you will not need them elsewhere.
Beware of helpers on the dock, even if they are employed at the marina. Having a hand on the dock is of great benefit, but only until they do something unexpected, whether rightly or wrongly.
You cannot dock too slowly. The reason for this is that you cannot hit the dock, or another boat, too gently. Slow down, using reverse if necessary, until the boat is moving no faster than is needed to maintain steerage way. Do not lose so much headway that you can no longer steer the boat. If the wind or current is moving toward the dock, let it push you in. If it is off the dock, plan to use it in the event that you must head off and try again.