[SalesForce] SOQL query problem, need assistance

You can't instantiate a List like that using an integer. You don't need to explicitly specify how much items are going into the list when it is created. Instead, just remove the integer:

//Instantiate the list
String[] myArray = new List<String>();

You're logic in your loop seems to be incorrect as well. Specifically, myArray's size will always be 0 when you instantiate it. Change your loop logic to:

for(Integer i=0;i<length;i++) {
   // code
}

You want to generate a List of strings based on the i variable. You will want to change your internal logic to:

//Populate the array
myArray.add('Test ' + i);

// Write value to the debug log
System.debug(myArray[i]);

Finally, you want to return that Array. You need to change your method so the return type is no longer void by changing its signature:

public static String[] generateStringArray(Integer length)

then it is just a matter of returning that array at the end of your method:

return myArray;

This would bring it all together as:

public class StringArrayTest {
    //Public Method
    public static String[] generateStringArray(Integer length) {
        //Instantiate the list
        String[] myArray = new List<String>();

        //Iterate throught the list
        for(Integer i=0;i<length;i++) {
            //Populate the array
            myArray.add('Test ' + i);

            // Write value to the debug log
            System.debug(myArray[i]);
        } //end loop

        return myArray;
    }//end method       
}// end class

Some more info on Arrays and Lists, Loops, and Class methods for future reference.

The following code will solve your problem:

public class ContactSearch {
    public static List<Contact> searchForContacts(String lastName, String mailingPostalCode) {
        List<Contact> retList = [SELECT Id, Name, LastName, MailingPostalCode
                                 FROM Contact
                                 WHERE LastName = :lastName AND MailingPostalCode = :mailingPostalCode];
        return retList;
    }   
}

Let's view it line-by-line:

public class ContactSearch

This is just a class definition. Public says that this class is in public scope

public static List<Contact> searchForContacts(String lastName, String mailingPostalCode)

This is method declaration. Again, public keyword makes this method public accessible(public scope).
Static means that this method is class-method(static-method) and not instance-method. Difference is that for instance-method you would omit static keyword, but you would need an instance of that class to call this method.
List<Contact> is return type of the method. It means that this method will return a List of Contact objects.
searchForContacts is method name, use it to call this method.
String lastName, String mailingPostalCode are parameters that are expected by this method.

List<Contact> retList = [SELECT Id, Name, LastName, MailingPostalCode
                                 FROM Contact
                                 WHERE LastName = :lastName AND MailingPostalCode = :mailingPostalCode];

This line assigns the result of the query to retList variable.
List<Contact> retList makes a variable with name retList of type List<Contact>.
SELECT Id, Name, LastName, MailingPostalCode FROM Contact is a simple query that will return all contacts. However, you only need the ones that match given parameters, so the next line of the query is:
WHERE LastName = :lastName AND MailingPostalCode = :mailingPostalCode
This adds a filter to the query, so that it will only return results which match LastName and MailingPostalCode with given parameters.

Note the : in the query before the parameters:

If you want to use variables in your query, they should have : in front. That tells your code that it's not the value to be searched for, but a variable from the code

return retList; will return the list of matching contacts that you just found

update: alternativly, instead of creating a retList variable, you could immediately return the query. Like this:

public class ContactSearch {
        public static List<Contact> searchForContacts(String lastName, String mailingPostalCode) {
            return [SELECT Id, Name, LastName, MailingPostalCode
                                     FROM Contact
                                     WHERE LastName = :lastName AND MailingPostalCode = :mailingPostalCode];
        }   
    }