Jarodemo:
What do villains want? Power.
Does he want to rule the land? Accumulate wealth? Control dark and terrible magicks?
Decide upon a general theme and build from there. Is the villain human or some other race? Does he work alone or have an army? Is he physically powerful or does he rely on wits, cunning and trickery is he in fact a she?
You don't need to get too detailed unless the party actually meet the villain, but it should give you some ideas for story lines.
Do you want a cookie cutter villain or one that is realistic?
Very few "real" villains are evil just for evil's sake.
Look at Hitler, all the acts he committed, though BEYOND vile, we done for the sake of Germany. They may have been ill thought out (declaring war on the US and the USSR); absolutely sick and stupid (his 'final' solution); and downright moronic (his idea of finding relics from a made up religion).
Look at Vlad Tepes, everything he did was in defense of Wallachia.
I can go on and on with real life villains. Just about everyone of them did it for a reason (barring the truly disturbed ones). Give your villains a reason for being bad.
The evil wizard in the tower outside town, the one that rules the land with an iron fist and punishes any infraction with a slow and horribly painful death? He does it because he's trying to protect the town and surrounding countryside from the Orc hordes that are slowly making their way towards the town. He has years to build up the strength of the settlement, but he knows he has to do it the hard way. He kills so brutally and publicly to keep the people in line. To focus them one following orders without question, something that will save more lives than he ends in the long run. He is forging the people into a strong nation that can defend itself against the coming horde.
The man brutally murdering and sacrificing people in a ritual manner? He found out that there was a demon that was getting close to escaping it's bonds. One that would ravage the entire countryside. The only way to strengthen those bonds was to brutally murder innocent people. The ritual of innocent blood spilled in a painful and particular way, repairs those bonds.
Then you have villains like The Joker. He's evil because of his insanities. He literally can not help himself. Is it right to murder him? Is it ok to kill one to save many? If it is, then my first two 'villain' examples might not be villains. The might be heroes. (This is why I don't agree with alignment systems.)
Not only do they make your villain believable, but they also offer plot hooks for future adventures.