We have to learn the irregular verbs list! You may find it useful to practice with this JClic activity created by José Hernández Portillo. It only has past simple forms:
Once you're good at it, you may want to go further and practice with past participle forms too. Another great English teacher, Alejandro Valero, has created these activities that you'll also find very helpful. They are in his wonderful site called "El Tinglado".
I hope all of them help you learn the irregular verbs.