Before explaining how to skate downhill, you should know that if you fall, it's not my fault! This is just a little disclaimer. You should also know that if you try any type of extreme sport, there's a possibility of getting hurt. Stay within your limits, and don't push them too hard if you're just starting out a new sport.
When downhill skating, it's also important to wear your safety equipment. Get a proper helmet; your every day skatepark brainbucket just won't help you at 50 miles an hour. You may want to invest in a good leather suit, too. Of course, if you plan on keeping your speeds down, then these won't be necessary.
Besides a helmet and leather suit, you'll want to invest in some slide gloves and a proper pair of shoes. When you want to slow down, you'll either need to drag your foot on the ground, or slide. Since you don't know how to slide yet, you'll probably be dragging your feet a lot. Get some good shoes.
The best way to start downhill skating is to just go out and try it. Find a hill near your house, preferably one that's not too steep or too long if you're just starting out, and then try and ride it. Wear your safety equipment! As you're going down the hill, you may want to carve back and forth. Start slowly at first, and work your way up to bigger, crazier carves. Carving will slow you down a little bit, too.
It's important to not just go straight down the hill. Since you haven't skated a hill before, you're probably not used to controlling speed wobble. You'll experience this quite a bit in downhill skating. Overcoming speed wobble can be incredibly difficult. When you first feel it, your natural instinct will be to tense up and brace for impact, but you'll want to do the opposite. Let your ankles loosen up, and try to ride out the speed wobble.
If you feel like you can't ride the speed wobble out, jump off and start running. This is a lot better than thinking you can handle the wobbles and then falling off. If you do fall off, try to roll. Rolling is better than scraping.
To control your speed, you'll want to slide. Put on your slide gloves, and find a hill. As you're going down this hill, try to push out the back end of the board by kneeling down, putting one hand (with the slide glove ON!) on the pavement behind you, and twisting your body towards the hand that's on the ground. The board should get thrown into a slide, and you should start to slow down.
Sliding takes practice, and there are many ways to do it. Don't forget that while you're skating downhill, you'll probably fall off a lot. Just try to control the speed wobble, and wear your pads. You should come out of your first run with a good experience.