There is more to creating a Shakespearean sonnet than the sonnet being fourteen lines long.
First, while Shakesperean sonnets are fourteen lines long, they must be written using three quatrains (four line stanzas) and a couplet (a two line stanza).
Second, Shakespearean sonnets have a specific rhyme scheme. The rhyme scheme for a Shakespearean sonnet is as follows:
abab bcbc cdcd ee
Third, Shakespearean sonnet (actually, most sonnets (including Spenserian and Italian)) are written using iambic pentameter. This is the meter of the poem. An iamb is a foot containing one unstressed and one stressed syllable (in that order). Pentameter means that there are five feet within each line (therefore, there are 10 syllables in each line). The meter of an iambic pentameter looks like this (u=unstressed and /= stressed):
u/ u/ u/ u/ u/
An example of an actual iambic pentameter meter is as follows:
u / u / u / u / u /
A little dog went running blindly on.
While Shakespearean sonnets can be hard to write, finding the meter helps greatly. A good place to start is the opening line. Once the meter has been identified, the rhyme and continued meter gets far easier.
One last element of the sonnet is the volta or turn. A volta is the place within a sonnet where the question posed in the first part is answered. Shakespeare placed his volta at line nine. Other times, the volta is found right before the couplet.
A link has been provided below which has multiple examples.