Yesterday, for the first time in my life, I was snowboarding. I liked it very much, and I got used to it very quickly, as I was skateboarding before. The only thing I did not understand was how to slow down. People tried to explain to me, but the only thing I could understand was that I needed to somehow turn the board horizontally. I tried to do it, but I just fell. I’m going to go snowboarding again next week and I would appreciate if someone clearly explained to me how to do it.
There are basically two ways to go slower while boarding.
First, point more across the slope, rather than down it. Easier said than done, sometimes.
Secondly, washing off speed by letting the board move slightly sideways, rather than nosefirst. This is the equivalent of snowplough. Let's say you're regular-footed, and riding heelside, so you're travelling to your left, across a slope which is going downhill in front of you. You roll your heels back, causing the edge of the board to dig more sharply into the snow.
Then push slightly with your right foot and twist your body left. The board is now turned slightly left from its previous angle, but still travelling in the same direction. The extra friction from the edge scraping in the direction of travel causes you to slow down a bit.
Like Jon Custer said, move your board so it's level (board not not pointing downhill) and then lean into the slope. I.e. you want someone downhill of you to be able to see some of the bottom of the board. (You can do this facing either upslope or downslope.) Think of it as trying to use the snow piling up under your board as the brakes.
Of course, the devil is in the details. You have to lean enough so that the front edge doesn't catch and you go flying down hill, but you don't want to lean so much that you fall into the hill. In theory, you can learn to bend your ankles to pitch the board to help ensure the edge doesn't catch and you don't fall.
I'll also add that I never mastered any of the above with a good amount of skill. I fell. A lot. And then I took up snowshoeing instead. :-)
The basic concept is to dig your edge into the snow. This has already been described by other answers. What is missing in these answers is the concept of weight distribution. The more weight you put in your front foot during a carving turn, the more the board will go downhill in proper carving line. If you put more weight on the back foot, the board will start to slide/scrape over the snow and slow you down. To come to a complete halt, put even more weight on your back foot to get the board orthogonal to the downhill line.
As a side note... You can achieve similar effects by turning your upper body and let the board follow. However, this means that your body is twisted and an unexpected bump will throw you off balance. Thus I strongly recommend to work with the feet