Why alkali metals and their compound are highly soluble?

You do have a printed periodic table, right? You can get the information online at www.chemicalelements.com/index.html but you really need your own printed copy to carry with you. It will be a primary reference for everything you do.

There is a diagonal line, B-C, Si-P, Ge-As, Sb-Te, and Po-At. Elements to the left of the line are metals, meaning they lose an electron easily, elements to the right of the line are non-metals, and elements on the line are semiconductors.

Alkali metals are all in the first column, meaning they all have one electron in their outside shell. Behaviors of elements are governed by electric charge and size. An atom with only one electron on the outside can lose that electron very easily. That is what we call "soluble".