Bike stem keeps turning while I'm driving.
 

Gahh, the whole headset turns. I know I have to tighten it, I just don't know how.

Perhaps take it to a bike shop.

A proper description of the parts would help someone help you. Is it an ahead set or a quill stem. What have you done to address the problem.

https://www.parktool.com/blog/repair...dset-standards

​​​​​​https://www.parktool.com/blog/repair...eadset-service

​​​​​​https://www.parktool.com/blog/repair...eadset-service

If you're not going to post any information at all about what you're working with you might as well just hit YouTube and find a video. It's way quicker.
ETA: I see this is your first post. Take a look around and check out all the different forum sections, like 'bicycle mechanics' which would be the obvious choice for your question, not GD. The people that run the forum included all of those sections so the forum works better. Use them.

If you have absolutely no idea what you are doing ..... consider paying someone and asking if you can watch. If you have a little aptitude, or confidence in your abilities, hit YouTube and watch a few different people do whatever you want to do.

Bike repair and maintenance is simple and easy ... and screwing up a bicycle while trying to fix or maintain it is simple and easy, too.

You said, "While I'm driving". Did you mean while riding or while the bike is on a rack while you're driving? If it's the former, the advice to see a mechanic is on point. If it's the latter, a bungy cord around the wheel and frame is the fix.