How do you form the past tense?

If it is an er verb you use the ending é

travailler- j'ai travaillé, ils ont travaillé, est-ce-que vous avez travaillé en France?

If it is an ir verb you use the ending i

finir- j'ai fini mais ils n'ont pas fini

If it is an re verb you use the ending u

boire- j'ai bu un coca-cola, mon père a bu un café

Most verbs use avoir as an auxilary the auxilary is the part that comes before the past participle so:

j'ai bu-

(the bold part is the auxilary and the underlined part is the past participle- that's why it's important to know the verb avoir by heart before you use the passé composé)

However some verbs use être as the auxilary, most of those verbs involve movement in some way.

A good acronym is:

MRS VAN DE TRAMP 

monter, restir, sortir, venir, aller, naître, descendre, entrer, tomber, retourner, arriver, mourir, partir 

je suis allée à la piscine- I went to the swimming pool (I'm a girl so I've added an e on the end of the past participle allé because when you use être as the auxilary the past participle needs to agree with the person  doing the action which in this case is me, and I'm a female)

je suis sortie de la maison de mes grandparents hier- I left my grandparents house yesterday 

Reflexive verbs (verbs with se in front of them) also use être as the auxilary like 

se réveiller- je me suis réveillé(e) à huit heures ce matin!

NB: lots of verbs can be made reflexive, when you do this remember that you need to use être and  make the past participle agree with the person carrying out the action