Remove text from String between characters which appear more than once

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.

You're calling String.valueOf() on the Event Sobject itself, not the Id string. As written, your first line should be:

EventIdString = String.valueOf( [select id from Event where Id=: (ApexPages.currentPage().getParameters().get('EventId'))].Id);

But you don't even need to use String.valueOf(). ID can be directly cast to String:

EventIdString = (String)[select id from Event where Id=: (ApexPages.currentPage().getParameters().get('EventId'))].Id;

And even that is superfluous! If your variable is already a string, you can simply:

EventIdString = [select id from Event where Id=: (ApexPages.currentPage().getParameters().get('EventId'))].Id;

Because IDs are strings. Or...you know:

EventIdString = ApexPages.currentPage().getParameters().get('EventId');

Works just fine too.