look, ive got a homework that consist on writing our lyrics over a song with a mexican rithym, and then play it tomorrow, but i dont have the chords for the song. can you help me?

the song is like this one: (couldnt find a better video ) http://www.youtube.com/watch?v=XzCbTxcupRM

1. wrong forum, like not even close

2. why do you need the chords if youre only writing lyrics
Am, G, F, E7

I may point out that these are of course not the chords of the song in the youtube-video of the OP (which seems to be rather a major progression in Bb, with the IV and V thrown in for a nice down-home song - play it with alternating bassline in a boom-chak rhythm and you're golden)

The Am, G, F, E7 -thing is another chord cliche that many people may associate with "latin" styles. I personally don't really associate it with 'mexican' music in particular, it is rather a spanish (flamenco, esp. rumba ) progression, or can carry over into styles like bossa and samba, depending on the rhythm pattern you play it with.

And rhythm really is the most important element here. Consider the video you posted: Harmonically, the chord progression itself isn't particularly noteworthy or carries any regional flavor - it all depends on the rhythm you play it with. The chords itself are used in similar ways in lots of folk music around of europe and european-derived music. What makes this 'mexican' though is rather the orchestration (what instruments are used), arrangements (what parts do the instruments play) and the rhythms (boom-chak 2/4, alternating basslines, etc..). If you understand and apply those things, your songs will really be able to carry a specific flavor.

If you're really in a last-minute hurry, just do a hack-job of taking chords/rhythms from other mexican songs you know (and if you don't know any, frequent tabsites, google, youtube, etc.), maybe change the order a bit if you want to make it more original, and make up a new vocal melody, then apply some nonsense lyrics about cats and icecream on top.

If you don't know what I mean with "boom-chak" alternating bass rhythm, it's basically something like this (here applied to a chord progression in A). This is especially meant as an approximation of that happy, folksy feel of the song in the video - if you want something more dramatic, go with a minor progression.

     A (I)                                          D (IV)                                        
     Q     Q    Q    Q       Q    Q    Q    Q       Q      Q    Q    Q       Q    Q    Q    Q     

  E7 (V7)                                          A (I)                                         
  Q       Q    Q    Q       Q    Q    Q    Q       Q     Q    Q    Q       Q    Q    Q    Q      

If you play with your fingers, you can strike the bassnotes with your thumb and strum the rest of the chord with the other fingers.

If you then manage to bring along an accordion too, you are golden. Hope that helps
i had to do the same thing in my spanish class but i acuatly wrote a song