Question

sodium ka formula Na Kyu Hota Hai

Answer 1

Yes,
It is that the symbol of sodium is Na
Although it starts with S then also it is Na.
If you will write S for sodium then it will be wrong as S indicates sulphur.

For the concept
I would like to inform you that the symbols for all the elements are decided from their scientific names.
They are not decided as per their names we use in present. Its scientific names.

The scientific name of Sodium is
$natrium$
From this natrium the symbol of sodium has been derived as Na.

It is similar like other elements also.
For example
Copper is denoted as cu because its scientific name is Cuprum.
Iron as Fe as its scientific name is Ferrum
And many more.

These all names are decided by the IUPAC for all the elements and other chemistry related terms.