How to survive on a desert island. Stay calm. The most important thing for you to do at first is to remain calm and try to think clearly. If you start to panic, you can lose control and end up forfeiting your chances of survival. You won't do that well if you've driven yourself crazy. William Golding's novel 'Pincher Martin' is a great example that illustrates how you can't do anything unless you 'take control', because the main character in the story lets panic get the better of him. Try making a "buddy" out of an object or animal near you, and talk to them to calm down. You should make 'safety, water, shelter and food' your main priorities, in that order.

Look around you and decide on some safety precautions. Start by asking yourself if the area around you is safe. Look around again and check if there are there any wild animals lurking in close proximity? Does it appear like there could be a flooding risk? The first important step is establishing whether the place where you are is physically safe.

Establishing the Necessities-

Look for clean and fresh water supply. Almost all people who are lost out at sea are found in a matter of hours or days. Science tells us that the human body can survive up to 2 weeks without food, but just 3-4 days without water. So if there is no natural source of fresh water, start looking for a way to collect rainwater.
Any source of water is great! As long as you have found a source of water, you can try to purify or desalinate it later.

If fresh water is available, try boil it for 2-3 minutes. This will purify it.
If you have a desalination device, that's even better! If not, don't fret, there are actually several methods by which you can desalinate water.
One such method is distillation. To distillate water, construct a solar still, or one that works over a fire.
A solar still can be built by filling a large, flat container with salt water or even urine which can be re-used, if the need arises. Place a smaller container in the center with a rock inside it to hold it in place. Cover it with a thin sheet of plastic or whatever similar material you have and place a rock in the center above the cup. If placed in direct sunlight, the water will evaporate and it will condense on the plastic sheet, which will then run down the sheet and drip into the smaller container.
For a method that works over a fire, create steam and condense the steam by using a large piece of metal or glass placed inside the steam. This will allow the condensed water to run into another container.

Create a shelter. You will need a shelter as protection from the elements and from natural predators as well. The best options are usually a natural shelter such as a cave, or one that you build.
The next step to follow after having found a natural shelter, is to build a more enduring shelter.

Music: SOS by Dhruva Aliman
https://dhruvaaliman.bandcamp.com/album/what-must-be