How do I check to see if a word contains a letter or group of letters?

I am working on a program for my CompSci I class and I'm entirely stuck. The assignment is to use if statements to check whether a particular string contains a letter or group of letters. I have a class created for this and I've laid out everything, but I just don't know how to begin searching for the characters. I'm really new to Strings and Return methods. I looked for similar questions already, but none really helped with what I was looking for. I would appreciate some help.

My code so far: Main:

import static java.lang.System.*;

public class Lab04e
{
public static void main(String[] args)
{

}
}

StringChecker:

import static java.lang.System.*;
public class StringChecker {

private String check;

    public void StringChecker()
    {
        check = "";
    }

    public void StringChecker(String s) //constructor
    {
        check = s;
    }

    public void setString(String s) //set string
    {
        check = s;
    }

    public boolean letterExists(char a)
    {
        return false;
    }

    public boolean findSubString(String s)
    {
        return false;
    }

    public String toString()
    {
        return check +"\n\n";
    }
}