Depending on your type of sailboat and the area where you sail, you might start out from a dock, a mooring, or a trailer on a boat ramp. Or you may carry or wheel a small sailboat in a hand cart down to the shore to start from a beach or other shallow area. In many resorts and other sailing areas, you might also rent a Hobie cat or other small boat that is sailed from the beach. If you've never sailed off a beach before, it can be tricky the first time. If you're just learning to sail, read more about sailing technique before trying to sail from a beach.
The more you know and think about before actually climbing onto the boat in the water, the more successful will be your experience.
Know the Boat Before You Start
First, become familiar with the boat's appendages below the hull: the rudder and centerboard or daggerboard. These are where new beach sailors typically get in trouble because you can't sail without them but often you can't start out with them fully down.
Beachable sailboats like a Hobie catamaran or a Sunfish usually have kick-up rudders. This means the entire rudder blade, or a hinged lower section of the rudder, can pivot up when the boat comes into shallow water. Check out this rudder mechanism on the boat you're about to sail so that you can get the rudder down fast as you move into deeper water. It may be as simple (on many Hobies, for example) as positioning the tiller handle in a way that the blade is lowered and locked in place. You may need to manually push the rudder blade down to where it "clicks" into place held by a spring or other mechanism. On some boats you may need to pull on a line that forces the blade down, and then cleat off that line to keep the rudder blade down. Get used to the mechanism and experiment with positioning the blade part way down, prepared to quickly lower it.
Second, become familiar with the boat's centerboard or daggerboard. Some boats, like a Hobie cat with double rudders, may not have an additional board to prevent leeway. Some other boats, like a Sunfish, have a daggerboard: a long, thin blade that is pushed down through a slot in the hull. The best designs have a spring or other mechanism that allows you to lower the daggerboard part way, without it falling all the way down. If your boat lacks such a mechanism, and the daggerboard can slide down freely in its slot, then you may have trouble getting free from the bottom until you're in deeper water. You can overcome this problem by affixing a bungee cord from the top of the daggerboard to the mast or a hull fitting: tension it enough that it pulls the top of board forward, keeping the board in place until you manually push it down.
A small sailboat with a centerboard usually has a simple line that you let out gradually to lower the centerboard gradually. Again, become familiar with its operation so that you can start out with the board just slightly down and then quickly lower it all the way.
How to Start from a Beach
If the wind is blowing onshore and is strong enough to make whitecaps and breakers, accept that it may be impossible to get off the beach unless you really know what you're doing. But with lighter winds and only small waves, you can sail off the beach with the wind from any direction.
Easiest is a wind blowing straight off the beach out toward the water. To sail downwind you don't need to use the centerboard or daggerboard, so it's easy to get started. Raise the sails with the bow pointed into the wind, and then pivot the boat around in the shallows and let the wind blow you out into the water as you lower the rudder in deeper water. You won't need to lower the centerboard/daggerboard until you turn to a different point of sail. Be careful not to accidentally gybe.
Starting with the Wind Along the Beach (Beam Reach)
Pick a starting point clear from obstructions and people in the water on the downwind (lee) side. Point the bow into the wind to raise the sails, and walk the boat out to a comfortable depth where you can lower the rudder at least part way. Trim the sails in part way for a beam reach and climb on the boat. Note that it will be blown sideways downwind (along the direction of the beach) until you get the board down, but at the same time the sails will be moving you out to deeper water where you can more easily lower first the rudder all the way and then the board.
Starting with the Wind Onshore (Against You)
This is the trickiest situation because you need to get the boat moving quickly out to deeper water, sailing close-hauled into the wind. Yet you can't sail into the wind without the rudder down for good steering, and you'll likely have to get the board down part way to start to prevent being blown sideways back onto the beach. After raising the sail, push the boat as far as you can out into deeper water, but not so deep that you can't quickly get climb on. Check the wind, and if it is somewhat to one side of dead onshore, plan to start out to the other side. Lower the rudder and centerboard as far as you can, remembering that you may be blown sideways or backwards, possibly into shallower water, before the boat starts moving forward. Trim the sails in but not too tight, and push the boat forward as you hop on.
If needed, paddle the boat out to deeper water before raising the sail.
Returning to the Beach
If the wind is directly onshore, drop the mainsail before coming too close into the shallows. Then gradually raise the board, as you approach under just the jib. Let the jibsheet fly just before reaching the shore, and coast in while you raise the rudder.
If the wind is along the beach, approach straight in on a beam reach and loosen both sheets to luff up and slow down as you reach the shallows, raising the board and rudder.
If the wind is offshore, approach at a 45-degree angle close-hauled until a couple boat lengths away and then turn into the wind and toward the shore, raising the rudder and board as you coast the last way in.