What Lens Should I Use?

Sometimes it is obvious - If I want to take something far away and I can't walk up to it, then I need a telephoto lens. If I wish to blur the background more, I also use a telephoto lens. Some creatures get spooked if you get too close so once again - use a telephoto lens
Landscapes - If I am taking Landscapes and I want to fit a lot into my image then I need a wide angled lens.
But what about other subjects like portraits, small things, buildings etc?

People - for portrait shots (head & shoulders) or kids you need a short to long telephoto lens 85mm to 200mm or more as this gives a nice face shape & you don't have to get too close. A wide angled lens distorts the face when you stand too close.
For full length or groups you can use a wider lens eg 35 -100mm.

Small subjects need a macro lens so you can get close enough to fill the frame. But macro comes in many focal lengths. A telephoto macro is better when you don't want to (eg scorpion or butterfly) or can't get close (dragonfly on a pond).
Sometimes a wide angled macro is better for seeing into flowers as you can get very close. You can get this with a compact camera.

Buildings also can require a range of different focal lengths. If you can get back far enough, a telephoto lens will avoid a lot of the problems of the edges not being straight. However, if you can't get back a wider lens is required. Use the least wide lens available to avoid distortion and don't have the edge of the building too close to the edge of the frame.

Wide angled Lenses 10 - 35mm
Mid Range (standard) 40 - 70mm
Telephoto lenses 70 - 600 + mm
Zoom lenses have many lenses in one. Check the range to see where yours fits