http://www.vatican.va/archive/ccc_css/archive/catechism/p3s1c3a2.htm
Faith is what leads you to salvation. Works are a result of a true faith.
You can not gain your own salvation by your works only, you can not even merit the forgivance of one single grave sin by works only.
Even if you would dedicate your whole life to helping others, spreading the Gospel, improving yourself etc etc but for some reason reject Jesus' sacrifice and God's truth and think that your actions are good enough to give you eternal life then you are wrong and will not be saved.
If you say you are a Christian and believe in God, but live a life full of sin, you are not repenting and are hurting others etc then it's a sign that your faith is false and dead.
16 Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles?
17 Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit.
18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.
19 Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.
20 Wherefore by their fruits ye shall know them.
>Matthew 7:16-20
So basically:
>Salvation comes through faith
>Works, while theoretically not strictly neccessary, are a natural result of a true living faith
This is the Catholic opinion. You can read it worded better, more accurately and in detail in the catechism. Don't let strawmanning protestants tell you otherwise.