Pronouns are used to refer to people and things that have already been mentioned. The main pronouns you need to be familiar with are:
a) subject pronouns (like yo, tú, él, ella, etc.)
b) direct object pronouns (like lo, las, me, te, nos, os, etc.)
c) indirect object pronouns (like le, les, me, te, nos, os, etc.)
d) relative pronouns (like quién, qué, lo que, etc.)
e) possessive pronouns (like el mío, la tuya, los suyos, etc.)
In order to successfully write with pronouns, you need to familiarize yourself with the different forms, the contexts in which they are used, and the order in which they appear. See the particular pages on each kind of pronoun for explanations.