What is the perfect tense and how do I form it?

When you want to say 'I have done something' in Spanish, the perfect tense is used. It is formed using the correct conjugation of the verb 'haber' (known as the auxiliary verb), plus the past participle.
Haber: (yo) he, (tú) has, (él/ella/usted) ha, (nosotros) hemos, (vosotros) habéis, (ellos/ellas/ustedes) han.
Then, for regular verbs, the past participle is formed by removing the -AR, -IR and -ER endings of the infinitives, and adding -ado for -AR verbs, and -ido for -IR and -ER verbs. For example, mandar becomes mandado, vivir becomes vivido and comer becomes comido. Examples: has mandado = you have sent, he vivido = I have lived, hemos comido = we have eaten.
There are quite a few verbs that have irregular past participles, but don't worry, once you learn the main ones you will start to notice patterns, and it will be easier to work out the participles for less common verbs.Examples: decir = dicho, volver = vuelto, ir = ido, abrir = abierto.