I've already checked many posts, but none of them solved my problem.
First of all your sketch is wrong. Any information that is not going to change should only be sent to the display once. You should remove all of the steps that you have in loop (leave loop blank between the curly brackets). If you really want to display "Hello" then do so in setup.
If that doesn't fix things up then you should start over.
Here are the steps that you should follow to get your display working.
(1) Get the backlight working, this typically involves only pins 15 and 16. Make sure it has a series current limiting resistor if it needs one.
(2) Get the contrast working, this involves pins 1, 2, and 3. You should get a blank display with the potentiometer at one end, and some really dark boxes on the top line of your display with the potentiometer at the other end. You should adjust the potentiometer until you can just barely see the row of boxes.
Do not proceed until this is working as described, if you do then you will just be wasting your time.
(3) Connect the LCD control and data lines to your Arduino. Do not forget to deal with LCD pin 5 or the display will not work, it is typically connected to GND.
(4) Load your sketch and see if it works.
If your display still does not work you are not going to get any really reliable help until we have more specific information. We need as much of the following as possible:
(1) A photograph that allows us to unambiguously follow each wire from one end to the other.
(2) A copy of the sketch that you are using.
(3) A link to the datasheet for the display.
(4) A description or photo of the results of running your sketch.