This is an important question. If you study the history of diseases, you will see that diseases spread rapidly at times causing much death and hardship. The reason for this quick spread is because there was a lack of quarantining people, who were contaminated. The most effective way to stop any spread is to isolate those who are infected from those who are not.
In view of this, the most appropriate steps to take is as follows.
First, you would want to locate what the disease is and how it spread. For instance, is the virus airborne, or does it need physical contact? Once this is determined, you can set a plan of action. Second, depending on the type of virus (and how it spreads), you will have to notify the people through all forms of media to get the word out. Third, from a more personal view, I would make sure that I wash my hands constantly. This might seem small, but it is rather effective. In addition, I would stay away from places where there are many people.
Finally, you will need to look for a cure and vaccine.
As a doctor will tell you, the first and foremost thing to keep in mind is to simply wash your hands frequently and thoroughly (but obviously that cannot alone make you safe from all kinds of viruses). As we saw in Asia when the SARS virus broke out, many people were seen wearing those surgeon-issue face masks to prevent breathing it in (if it were an airborn virus) so that is not a bad idea. Do not touch things that are touched often by the populace (EX: crosswalk buttons, door handles at a store, public bathroom knobs, ect). As seen with the Bubonic Plague, the cities of Europe with the highest population density were the ones devastated the most, so consider where you live and how much people surround you/come in contact with you. Theres a story in Giovanni Boccaccio's "Decameron" where a group of city dwellers flee to a church on the rural edges of town to escape the Plague, so keep that in mind. Of course, in those medieval of times, the people had no idea what was really causing this outbreak; they blamed divine wrath, and black hooded creatures to be behind it, which brings me to another important point: RESEARCH AND UNDERSTAND THE VIRUS. Your best defense is to understand if it is spread through 1) The Air 2) Food 3) Personal Contact 4) Contaminated water 5) Zombie bites ;) ect, to find how to best defend against it. Knowing how the virus spreads and how it works, as well as early symptoms is your best weapon.
Epidemics happen. And our society is overdue for the next Black Plague to terrorize humanity, and you have already done what most people never do: you have looked into what you can do beforehand!