I am puzzled. How to decide which MM lens to use in which situation? I usually want to shoot landscapes and wildlife. Which lens is suitable for these specifically?
Most people associate wide angle shots with landscape photography. However, here's a whole tip article on using a telephoto for landscape.
Most people associate telephoto shots with wildlife. However, here's a whole tip article on using a wide angle for wildlife.
You'll get more results and examples by searching for "telephoto landscape photography" and "wide angle wildlife photography."
As far as the focal length is concerned, there is no one best for anything. Photography is an art form and, as such, the product of a shot is whatever you want it to be. Now, if you want it to be wide, then you'll use a wide angle lens. And if you want it to be close, like shooting a headshot on a lion, then you'll need to use a telephoto lens.
At the end of the day, use whatever focal length you need to get the shot framing that you want. If you run out of focal length, then physically move closer or further away until you get the framing that you want.