How do you conjugate perfect tense?

Most verbs use the auxiliary 'avoir' verb along with the action that they did in the past. You have to conjugate this according to the person/people speaking, so 'I' would be 'J'ai', you (tu) would be 'tu as', he/ she is 'il/elle a', we is 'nous avons', you plural is 'vous avez' and they is 'ils ont'. These all mean someone has performed an action in the past. Now, along with these you add the action that they performed, again conjugated to fit the past tense. Most verbs either take é, i or u, for er, ir and re verbs respectively, for example 'croire' would be 'cru'. Altogether, 'I believed' would be 'J'ai cru'. Exceptions must be learnt off by heart unfortunately, I can provide a list for you to learn, but some of the main ones are 'faire = fait', 'avoir=eu' and so forth.
The remaining verbs use 'etre' instead of 'avoir'. The main ones that go by this rule can be remembered in the word 'MRS VANDERTRAMP'. Each letter stands for the verb: Monter, Retourner, Sortir, Venir, Aller, Naitre, Devenir, Entrer, Rentrer, Tomber, Revenir, Ascendre, Mourir and Partir. You have to conjugate etre to suit the past tense as well: I = Je suis, You singular= tu es, he/she = il/elle est, we=nous sommes, you plural = vous etes, and they=ils sont. You pair this with the conjuagated action verbs- which must also be changed to suit the gender of the person doing the action, the amount of people as well as the past tense, eg. 'she went to France' is 'elle est allée en France', the extra 'e' making it feminine.