Generally, bad road conditions are caused by one of three weather events: rain, snow or ice. (If your car is struck by a tornado, we really can't help you.) Let's look at those one at a time:
Rain: If the weather has been dry for a while, a layer of oil can build up on the road which, when combined with a sudden rainstorm, can make the road dangerously slick. The good news is that the oil layer washes off fairly quickly, but the first few hours after rain starts falling are the most dangerous, because the oil is still there. Try to drive more slowly than you usually would and avoid braking suddenly. In particularly bad rainstorms, where the road surface is coated with a layer of water, a very hazardous condition can occur: hydroplaning. This is when the water builds up so quickly under your tires that your car literally starts to float, driving on top of the water instead of the road. Your brakes won't work normally while hydroplaning because you no longer have traction. The steering wheel will no longer make your car turn correctly. Avoid hydroplaning by keeping your speed down, but once it happens take your foot off the accelerator to let the car slow down, don't brake hard because it can cause a skid, and drive in a straight line. You have the most control when moving in a straight line, so point the wheels straight ahead. If you turn the wheels at an angle to the direction in which you're traveling, you're likely to go into a dangerous skid. Okay, it's easy to say that you should go straight when you're heading toward an obstacle, but you'll be surprised at how fast you gain control again as you slow down and the brakes start working.
Snow: Your worst problem on a snowy day may be getting the car moving in the first place, especially if the snow plows haven't gone to work yet. If your car won't move out its parking place in front of your house, it's probably telling you to stay home. Otherwise, the rules are similar to those for hydroplaning. If you start skidding, let up on the accelerator, try to drive in a straight line to get control back and never, ever try to go fast on a snow-covered road. If you don't have antilock brakes, pump your brakes rather than tromping on them. If you do have ABS, this will be done automatically.
Ice: Perhaps the most treacherous weather condition for driving is freezing rain. This is when supercooled raindrops -- water that's been cooled below the normal freezing point of 32 degree Fahrenheit -- hit the road surface and freeze instantly, turning the highway into an ice-coated skating rink. Avoid driving under these conditions if at all possible, because it's almost impossible to drive on a slickly frozen road surface. If you absolutely have to, drive as slowly as you can. Brake gently and long before you actually need to stop. Follow the hydroplaning rules and try to drive in a straight line if you feel your car starting to skid (though on an ice-coated road, it's surprisingly hard not to skid). Watch for slippery spots on the road surface, but bear in mind that you won't always be able to see them. So-called "black ice" is effectively invisible and you won't know it's there until you go into a skid, so in freezing rain just assume the road is dangerously slick unless you have compelling evidence to the contrary, like cars driving ahead of you melting the ice with the friction of their tires.
Perhaps the best rule for driving on icy roads is to stay home with a warm drink and wait for the thaw. You may miss the movie you wanted to see or a party with friends, but you and your car will weather the storm in good condition.