Ricci Adams' Musictheory.net. The Rhythm Trainer. Think you know Music ? See how much there is you didnt know! Hmmm...Well you've got to use words like you would notes, but how you put them together is up to your own 'inspiration' I suppose.
And like the theory of music there are certain grammatical and rhythmical ways of writing that create lyrics. But this is off topic.... I believe you also have some lessons posted in the forums Denis. Music Theory.