How do I change my villagers hair and skin?

You must go to Shampoodle. Spend 8,000 bells within the first 10 days of the game at the Able Sisters, then 8, 000 bells withint the first 10 days Kicks has opened.
You cannot change skin color, but you can change hair and eye colors.

The other answer covered changing hair well enough, but not skin color.

You can get a tan in-game. It is easiest to get if you visit Tortimer's Island. You need to spend long amounts of time in the sun (either in your town or on the island). The tan should appear the next day, and will fade over time. You can keep the tan if you spend enough time on the island during the day, or if you spend enough time outside in your town on a sunny day. There are several levels of tan, starting with very pale and ending with very dark.

To change your hair colour you must go to Shampoodle and pay 3,000 bells for a haircut. This includes a cut, colour and new style.

To get a tan, if it's Summer in your town, stand outside without an umbrella or a hat for a few hours. Your characters skin should be a bit tanned. If it isn't Summer, go to the island and stand outside without wearing a hat for a few hours. Your character should be a bit tanned.