Wall, The

I decided to paint the inside of my house but I'm not sure if I have to remove the old paint before putting the new one. I know I have to clean the wall first so the new paint can stick better. Can anyone tell me if I need / should remove the old paint before painting a new color? And if so, what is the best way to do it? I'm sure it will be a difficult process in removing the paint. Thank you in advance.