How can I paint a wall professionally?

https://www.youtube.com/watch?v=3HSuqwi_CY8

https://www.youtube.com/watch?v=GjIhG62-J30

https://www.hometalk.com/diy/paint/walls/learn-how-to-paint-a-room-like-a-pro-with-these-7-tips-and-tricks-40635647

https://www.hometalk.com/categories/how-to/paint/walls

Hi Elizabeth, preparation is important, here is a video with pro tips for you

https://www.youtube.com/watch?v=iCIZTXkIweg

It is more in the prep than in the actual painting. I didn't go through everything that is already posted but I will give one tip that a lot of people don't know. Wipe the walls down with a plain swiffer cloth to remove any dust.

A professional paint job is all in the prep. Tape off the edges if you aren't comfortable with a paint brush. Fill in holes with spackling and then sand it down smooth. Remove dust and grime from the wall before painting. If you are making a dramatic change, use a primer tinted to the color.

Also, be sure to test the paint on multiple walls in the room to be sure it's the right color for the room. Paint can look completely different in different rooms of the same house.