Irregular verbs are those verbs that do not take the -ed ending for the Past Simple and Past Participle forms. They are very frequently used and, unfortunately, must be learnt by heart ("de memoria").

Here you are two files with lists of the most common irregular verbs in English (it is believed that there are 618 irregular verbs approximately in English). In the former, the irregular verbs have been divided into blocks from I to XV so that you can find it easier to learn them according to your course. In the latter, the same irregular verbs have been ordered alphabetically.
Finally, if you want to check whether you have learnt them properly or not, you may try to fill in a couple of crosswords on the web. The former includes verbs from a-l, and the latter from m-w. Or, do you prefer to answer these 85 questions?

Have fun with irregular verbs!