How do I form the perfect tense?

In Spanish, the perfect tense corresponds to the English construction to have done something e.g. I have eaten lunch. Like all compound tenses in Spanish, the verb used to form the have/has part is haber. You also need the past participle, which in the above example would be eaten.

Conjugations of haber

(Yo) he - I have

(Tú) has - You have

(Él/ella) ha - He/she has

(Nosotros) hemos - We have

(Vosotros) habéis - You (pl.) have

(Ellos/ellas) han - They have

Usually, past participles are formed by taking the ending off the infinitive of the verb, and adding -ado (-AR verbs) or -ido (-ER/-IR verbs).

Examples

To arrive - llegar / arrived - llegado

To eat - comer / eaten - comido

To go - ir / been - ido

There are also some irregular past participles, i.e. ones that don't follow the -ado/-ido rule. 

Some common irregular past participles

To say - decir / said - dicho

To write - escribir / written - escrito

To do - hacer / did - hecho

To see - ver / seen - visto

Bringing it all together...

We have arrived - hemos llegado

You have seen - has visto

She has eaten - ha comido

Once you know the conjugations of haber and how to for past participles, the perfect tense is pretty straightforward!