To conjugate irregular verbs in Spanish, you need to know the type of irregularity they have. Such irregularity affects the first part of the verb, the root.
Before learning to conjugate irregular verbs in Spanish, I would recommend you learn to conjugate the ones that are considered regular. Visit the post called How to Conjugate Regular Verbs in Spanish to do so.
If you already know how to work with regular verbs and would like to learn to conjugate some irregular verbs, visit the post called How to Conjugate Stem-changing Verbs in Spanish.