Pray to the Holy Spirit for the wisdom to know your sins and the courage to confess them. Take a look at an Examination of Conscience for guidance.
Start with the sign of the cross and tell the priest how long it's been since you last went to confession. If it's been a while, that's okay, just let the priest know.
Name all of the sins that come to mind based on your examination of conscience.
Listen to the advice the priest gives you based on your confession. Remember the Penance he gives you, which is to be completed after your confession.
Pray the Act of Contrition. There are a few versions. Use whichever one you remember. If you have forgotten, don't worry. Tell the priest and he will help you through it.
Be attentive as the priest prays the Prayer of Absolution. Respond, "Amen", and rejoice in knowing that your sins are completely forgiven.
The priest will conclude with, "Your sins are forgiven, go in peace", or similar words. Respond, "Thanks be to God!"