I live in the tropics and our water supply have regular interruptions so I need to install a water storage tank. I am not sure if my connections are correct. Can somebody look at my sketch and give advise?

Post your sketch.

Bummer, I cannot past the sketch here, I did post it in a second question though.

I'm not smart enough to draw on a computer but here is what I would do. First, you need to install a shutoff valve BEFORE the connection for a tank. This shuts off any water back feeding to the city water. You should also have a backflow preventer at the valve. Then, I would install a Tee and in the side of the tee, install another valve and then go into the tank. I like 1/4 turn ball valves. To start, open all the valves and let the tank fill. Then close the tank valve. If the city water fails, the check valve will keep the water in the tank but you would also close the city supply valve. Hope you can understand.

Might want to put a float valve on it to keep from having to monitor tank level.

https://www.grainger.com/product/GRAINGER-APPROVED-Pipe-Mount-Float-Valve-with-6AMA0?breadcrumbCatId=4926&functionCode=P2IDP2PCP

Not sure what your water source is but here is a basic system you can install, or check your own design with.

https://www.practicaldiy.com/plumbing/water-supply/fitting-cold-water-tank.php

Thank you all. My question is partly answered but I am still not sure about the check valves.Here is the idea: I take the street water supply through a shut off valve to a float valve to fill up my tank at all times. At the same time I connect this inlet to the house to have pressure when there is street water. When street water fails, I have gravity pressure from the tank.However, I think I need a check valve on the tank outlet to keep the tank from filling via the outlet when there is pressure and also a check valve on the inlet pipe to keep the water running back to city.I would appreciate comments.

You only need a check valve on the inlet from the source, not the outlet to the house. You don't worry about back flow from the house faucets when you shut them off, as only FLOW is affected, not pressure, and certainly not from a COLD water gravity fed only tank. Just make sure the inlet check valve is installed in the correct position to allow flow to the tank.

Yes, a check valve should be installed on the supply side from the city. Make sure the arrow points in the direction of the flow. See the arrow on the right side thread fitting.
https://cdn.mscdirect.com/global/images/ProductImages/4588440-21.jpg