You said you know the rhyme scheme  of 14 lines (ABAB CDCD EFEF GG) and it's in iambic pentameter (ta DA ta DA ta DA ta DA ta DA).

You will have three stanzas of four lines, followed by a concluding couplet. The final couplet should be some big statement, perhaps about a future relationship or a certain final feeling. The three stanzas could be focused on what specifically about him made your heart stop, what characteristics about him attracted your attention. These can be elaborate description of his walk, his eyes, his hair, etc. Or you can focus on your own emotions for these stanzas.

Don't forget you can do some word inversions (changing the order of the words) in order to keep the meter or rhythm correct, as well as the rhyme.

Check out some examples of Shakespeare or Elizabeth Barrett Browning (especially Browning's "Sonnets from the Portuguese").