Break the problem up into separate steps and do one step at a time.
For example: prompt for a sentence and read in the sentence and print it out.
See the Scanner class for a way to read in a sentence. Many code samples on this forum if you use Search.
public class LetterCount
public static void main (String  args)
int freq = 0; // frequency of charactersdeclared and initialised to zero
String lineTwo ; //string called lineTwo initialised.
System.out.println("Enter a string of text;\n"); // prompt user for the a string of text.
lineTwo = input.nextLine(); // read text from user.
System.out.println ("Enter letter"); // prompt user to enter a letter
// in which they would like to find
// out how many times its appearing.
String letter = input.nextLine(); // get line of text from user
for (int i = 0; i<lineTwo.length(); i++) //iterate through the line of text up the length of the text entered.
if (lineTwo.charAt(i) == letter.charAt(0)) // compare the character at i
// from the first line and character that the user inputs
freq++; // increment the frquency by 1 if the comparison in the of statement is equal.
if ((lineTwo.length()-1) == i) // compare
System.out.printf("%s \t %d", letter, freq ); // print the letter and how many times its appearing.
System.out.printf("\n\n"); // print two lines.