how to get input from the user and save that into a text file

In C you have stdio.h which contains all standard io functionality:
In your scenario you would require:
printf() to write output to the screen.
scanf() to read input from keyboard.
For file operations you require a couple more steps:

FILE *myfile;
myfile = fopen("myfile.txt", "w"); // or r, a, r+, w+, a+
fprintf(myfile, "Whatever...\n");
fclose(myfile); // always

Resp Person,
Program Accept 20 lines, to change line change loop count.
After press enter button, that line gets write to file.

-----------------Try This code-----------------------

import java.io.BufferedReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;

public class Demo 
{
	public static void main(String[] args) 
	{
		try 
		{
		      //create a buffered reader that connects to the console
		      BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		      //create an print writer for writing to a file
		      PrintWriter out = new PrintWriter(new FileWriter("output.txt"));
		      		      
		      for(int i =0; i < 20; i++)		      		     
			      //output to the file a line
		    	  out.println(in.readLine());
		      
		      //close the file (VERY IMPORTANT!)
		      out.close();
		   }
		   catch(IOException e1) 
		   {
		        System.out.println("Error during reading/writing");
		   }
	}
}