For Dandruff problems
Do a vinegar rinse to balance your scalp to start with. 1 cup of apple cider vinegar (or white, I find apple is a bit better) to 8 cups of water: soak your head in a bucketfull for a couple minutes, then take your head out and let the vinegar sit there for about 3 - 5 minutes, rinse well.
If your scalp is really itchy, apply a small amount of tea tree oil to your scalp at night, about an hour before bed so it has time to absorb a bit before laying on a pillow. Doing this between washings can really help with scratching, and calm down your scalp.
If these aren't helping out as you thought they would, you can try an over the counter Shampoo such as Neutrogena T Gel.
If nothing is helping, go see your physician to make sure that there's nothing serious going on with your skin - if there is a more serious problem your physician May direct you to a dermatologist.