There are several reasons one should know how to write a sonnet in preference to other poetic forms. The sonnet has enough rules about structure and rhyme to make it nicely disciplined and self-contained. On the other hand, the rules are not so difficult that writing a sonnet is hugely laborious. It's long enough to say something meaningful, but short enough to keep the reader's interest.
To know how to write a sonnet, you need to know its most recognisable characteristics:
1. Fourteen lines long
2. A rhythm - in English, usually 5 beats to a line, da-DA da-DA da-DA da-DA da-DA, like Shakespearean verse
3. A 'break' at the ninth line (ie there should be a change in tone or imagery or argument between what's said in the first eight lines and what's said in the final six)
4. A regular rhyme-scheme which reflects the sonnet's basic structure. Italian sonnets usually go abbaabba cdecde or abbaabba cdcdcd. English sonnets, as popularised by Shakespeare, often go ababcdcd efef gg.
Knowing that, here's more on exactly how to write a sonnet:
First, choose your sonnet's topic - not the title, but the subject matter. Here's a useful secret: make it specific. Instead of 'Love', narrow it down to 'How much I love you on Tuesdays', or 'My childhood love for chocolate ice-cream', or 'The way love goes wrong when you forget anniversaries.'
Then, plan your structure a little. A sonnet describes a movement in one's thoughts or feelings, not an unchanging state of mind. What you say in the first eight lines is going to shift and develop from the ninth line onwards, and the concept of the whole sonnet may then also be summarised in the final two lines. Scribble a few notes about how you feel the idea should grow.
Lay out your idea or argument in a fourteen-line structure. Don't worry about rhymes yet. Then, start fitting rhyme and rhythm to the ideas, and, vice versa, tweaking the phrasing, position and emphasis of the ideas to bring in the rhymes and rhythm you want. The title comes last.
For a much more detailed description of the above method of how to write a sonnet, including an example of a sonnet being developed from first concept to final form, see this page on how to write a sonnet - which, by the way, will also show you how to write your sonnet in calligraphy, for even more impact on the reader.