How do I fix this?

If the steering wheel is shaking pretty bad at any speed it is a safety concern. I would not recommend driving the car until you can have a service professional inspect it. The cause can be anything from improperly inflated tire pressures, tires being out of balance (spun wheel weight), to damage. Something could be bent. Either way, if the cause is not obvious when you visually inspect you should get the car to a qualified mechanic that can determine the cause for you.

Typically wheel and tire balance issues come in around 50-60 mph. Vibrations that occur outside of that speed range could be.anything else. Start withhaving your tires rebalance and reassess the issue. Take out to your local shop that you trust and they should be able to get you going back onthe road SAFELY.