How can you know which virtue is the one you need to work on? Know yourself and you will know what virtue you are in need of most. Now it is true that we should practice all the virtues, and strive to live them all. But it is also true that we have a natural inclination to some, and a complete rejection towards others.
In order to know which one you really need to work on you have to examine yourself, your conscience and sins. This will guide you to finding your dominant fault. Fault? Why do you have to know your fault if we are examining virtue? In order for a doctor to cure someone, restore them to health, they have to examine the symptoms of the illness. The same idea applies to the spiritual life. In order to know what virtue you are in most need of practicing, you have to see what defect or fault you are more inclined to committing. Almost all our sins have some trace of our dominant fault (vice). You have to be willing to examine yourself with sincerity, and have courage to call the vice by its ugly name without excuses in order to know the right virtue you need to conquer it. Every vice has its opposite virtue that if you implement will help you overcome your fault.
We must be conscious that the fight is never over. Although you manage to win little battles or overcome great struggles, your dominant fault always remains and will be constantly trying to rise again within you and conquer. This life is an endless battle. We must seek to persevere in the struggle until the end.
Here is a list of vices and their opposite virtues.
Lust (excessive sexual appetites) vs. Chastity (purity)
Gluttony (over-indulgence) vs. Temperance (self-restraint)
Greed (avarice) vs. Charity (giving)
Sloth (laziness/idleness) vs. Diligence (zeal/integrity/Labor)
Wrath (anger) vs. Forgiveness (composure)
Envy (jealousy) vs. Kindness (admiration)
Pride (vanity) vs. Humility (humbleness)