Basic rules
1. Write from left to right, and from top to bottom 
As a general rule, characters are written from left to right, and from top to bottom. For example, among the first characters usually learned is the number one, which is written with a single horizontal line: 一. This character has one stroke which is written from left to right.
The character for "two" has two strokes: 二. In this case, both are written from left to right, but the top stroke is written first. The character for "three" has three strokes: 三. Each stroke is written from left to right, starting with the uppermost stroke:
This rule applies also to more complex characters. For example, 校 can be divided into two. The entire left side (木) is written before the right side (交). There are some exceptions to this rule, mainly occurring when the right side of a character has a lower enclosure (see below), for example 誕 and 健. In this case, the left side is written first, followed by the right side, and finally the lower enclosure.
When there are upper and lower components, the upper components are written first, then the lower components, as in 品 and 襲.
2. Horizontal before vertical 
When strokes cross, horizontal strokes are usually written before vertical strokes: the character for "ten," 十, has two strokes. The horizontal stroke 一 is written first, followed by the vertical stroke → 十.
3. Cutting strokes last 
Vertical strokes that "cut" through a character are written after the horizontal strokes they cut through, as in 車 and 中.
Horizontal strokes that cut through a character are written last, as in 母 and 海.
4. Diagonals right-to-left before diagonals left-to-right 
Right-to-left diagonals (ノ) are written before left-to-right diagonals (乀): 文.
5. Centre verticals before outside "wings" 
Vertical centre strokes are written before vertical or diagonal outside strokes; left outside strokes are written before right outside strokes: 小 and 水.
6. Outside before inside 
Outside enclosing strokes are written before inside strokes; bottom strokes are written last: 日 and 口. This applies also to characters that have no bottom stroke, such as 同 and 月.
7. Left vertical before enclosing 
Left vertical strokes are written before enclosing strokes. In the following two examples, the leftmost vertical stroke (|) is written first, followed by the uppermost and rightmost lines (┐) (which are written as one stroke): 日 and 口.
8. Bottom enclosing strokes last 
Bottom enclosing strokes are always written last: 道, 週, 画.
9. Dots and minor strokes last 
Minor strokes are usually written last, as the small "dot" in the following: 玉.
SOURCE: Wikipedia
