There are 8 basic strokes used to write Chinese characters. These strokes can be combined into more complicated shapes. Here is an overview of the 8 strokes, which can be combined to write a single character, 永 (yǒng, "eternity"):
Of these strokes, the most important to get right as a beginning learner of Chinese are #2 above, the horizontal stroke (which you'll notice is slightly angled up toward the right), and #3, the vertical stroke (which you'll notice is precisely vertical, without any angle to either side). Make sure that your horizontal strokes are clearly horizontal, and that your vertical strokes are exactly vertical.
Also worth noting is stroke #4, the hook. Be careful to note when this appears at the end of a stroke when you practice writing characters. Omitting the hook is like writing an English "i" without adding the dot. It's just wrong, don't do it.
Please also take a look at this supplementary video on the basic writing strokes of Chinese, from the ChineseFor.Us Youtube channel: