The most common mistake that domino "newbies" tend to make is to leave too much space between the dominoes.
In order to set them up in the correct distance, most domino-builders use a little helper - the "template", also called "fork" or "comb". With its help, the dominoes have exactly the perfect distance beweteen them: the gap should be just as big as a domino is thick. Another advantage is that using the template, you can build perfectly straight lines in fields. That is especially important if two lines are supposed to fall in opposite directions without interfering.
Another basic technique are split-offs. They are used when one line is supposed to topple several ones. You start with a line in which one domino is shifted to the side by half its wideness. It now sticks out of the line like a thorn. That domino can now be the start of a new line - and that method can be repeated as many times as you want.
Now that you know how to correctly set up the dominoes, you can go on to the second to most important task - building turns, because of course you won't always building only straight lines. There are many different turns. The shortest possible ones - the ones with the highest risk of failing, too - include only three dominoes in a 90° turn. Of course, there's no limit to how many dominoes a turn can include, but the standard is 10 dominoes for 90°. With a template, you build a line and split off a second one in order to start the turn. Now lie down two dominoes next to the first domino in the turn. Add two dominoes turned by 90° to those dominoes. At the end of those dominoes, you place the domino that will end the turn.
In many domino course, you will need to cross two lines. The technique you see on the photo is the best suitable one and works with a 100% safety.
After learning these basics, additional tricks are waiting for you here.