One of the best driving safety tips you can learn is to take a moment to correctly align all your mirrors.
Simply line the rearview mirror up so you are looking straight out the back window. Frame the window in the mirror.
Sometimes you position yourself differently in the seat. And if you share the vehicle with other drivers, they will probably have to adjust the mirror. So make appropriate adjustments each time you get in the car.
At night you should flip this mirror upward with the lever on the bottom. If you align the mirror properly during the day, flipping the lever will put it in proper position for the night.
Adjust the passenger-side mirror so you can see just the tiniest bit of the car's side, and straight backward from that. You need that little edge of your car for your brain to know how everything else you see in the mirror is oriented.
Make sure the mirror isn't tipped too far up or down, as well.
Make sure you can see a sliver of your car and the road straight back. Even with this mirror correctly aligned, you still have a "blind spot"; you won't be able to see cars that are slightly behind and to the side of yours. No amount of adjusting can remove the blind spot.
Some rearviews come with a small fish-eye mirror that can help with the blind spot. But the most effective way to deal with it is to briefly turn your head and look for yourself before you change lanes.