Definitely invest in a good shampoo and conditioner. This can vary person to person, so there's no quick pick that fits for every individual. You'll want to shampoo at least once a week, as stated yesterday. If you're hair is oilier than most, shampoo at least 2 times per week.
Then, if you're natural or relaxed invest in a great natural oil. If hair is super dry, then use a little on the scalp. A bottle with an applicator tip works best to gradually apply oil. Just a bit is needed in palm of hand depending on length and texture.