As for my assignment I had to write a program to swap 2 digit number
Example:- If user input 12 the swaped number Would be 21.
Along with ALgorithm, and a short description on this program(As how will you solve this program or what measure you'll take to get desired results).
Please Help..

Have you started already? If not, why so? What in particular are you having difficulties with?
As a hint: modulo 10, division by 10, multiplication by 10; you can pretty much solve your task using these operations.

a piece of advice: try to work the digits separately.. and listen to the hint above!

Is this correct...

void main();
int a,b,c;
printf("Enter a 2 digit number\n");
printf("The swaped digits are %d%d",b,c);

Is this correct...

Apart from not following the standard and being unportable I'd say no.
Did you even try it out? Compile, run and answer your own question.
If it doesn't compile, fix the compiler error, if it runs but doesn't produce the correct output, fix the bug and compile again.