I have a 2001 Chevy malibu...I put 10W-30 oil in my car by mistake. I just saw where it says use 5W-30. What's going to happen to my car and what can I do??

Newer designs on engines use metering passageways and/or orifices to control flow to critical places inside engines. The old design used pressure to flood the bearings with oil. The controlled flow process turns out to be a better idea. So o o o, you vary oil flow via different sized orifices and oil viscosity. Just one viscosity range difference isn't necessarily going to kill your engine, but it's better to use what the engineer thought was best for that engine at a certain temperature range.

5W-30 means the oil that works best in Winter ( the W means winter weight) is 5W-30. In really cold weather, that will make a difference, but otherwise, you won't hurt the engine in warmer weather. 30 is the heavier weight oil for summer. The viscosity index 10W-30 gives a heavier oil for very cold days than 5W-30 would give. Unless you are starting in sub-zero weather, relax.