Your car antenna is more than a decoration. Without it, your radio may as well be turned off. There are generic replacements, but it is usually better to buy one specifically made for your vehicle.
It's best to purchase a small inexpensive tool kit. These are about the size of a book and have just about every tool you would ever need for simple repairs. Keep this in your trunk so it will always be handy.
Most antennas connect just by screwing onto the base. The part that is usually bent or broken is called the mast. It will have a space near the bottom that is made to fit a wrench. Unscrew the old one and replace this with the new one. Tighten it snugly but not too tight: it should be just enough so it can't be turned with your bare hands.
For more complicated replacements, you may have to remove the base. Once the antenna is unscrewed look closely at the cover on the base. Most will come loose with just a wiggle. Others have small screws that have to be removed.
When you remove the base, it will have a cable coming from inside the car. On some the cable is permanently attached. On many it is made to be easily unplugged. Plug in the new base and mount it to the car. Then attach the mast or antenna.
If the antenna cable is permanently attached to the base, you will have to route it to the radio. Follow the route of the old one. This may require removing some dashboard panels. When you reach the radio, unplug the old antenna cable and plug in the new one. Test the radio before putting everything back together.
On older cars and on cars with automatic antennas, the procedure is a bit different. These types of antennas are usually mounted with some, but limited access, to the backside of the antenna mount. They are usually mounted on top of the front or rear fender; there are often large screw-on devices and spacers located on the backside of the fender area. These devices can usually be hand tightened. If the old one will not come loose, you can use a large pair of channel lock pliers to loosen it.
An automatic antenna raises and lowers as you turn your radio on and off. I would leave replacement of this type of antenna to the professionals unless you have the ability to install a car radio.
Other than the electrical connections the procedure is pretty much the same as above. The antenna or mast is replaceable on most models.
With a little research and some patience you can enjoy your radio again.