Lighting, Light Fixtures, Ceiling and Exhaust Fans - Wiring Under a Sidewalk

I am trying to light up one of my trees but I have to go under a side walk. How do I do that? Is there a special tool to dig?

There are a number of possibilities: (1) A garden hose connected to a piece of conduit usually goes under easily after you turn the water on. (2) Sometimes you can just pound the conduit under. (3) Worst case you can use a trenching shovel.

Home centers sell a fitting to attach a garden hose to a piece of black iron pipe.

If the ground is soft or sandy, you can drive the pipe under the sidewalk with a sledgehammer (put a coupling on it first to protect the threads). If you can't drive it all the way under, attach the hose and "jet" it the rest of the way. Use the piece of black iron to pull in whatever you are using to protect your underground wiring (usually PVC), and you're good-to-go.

How do you turn the right angle?

What right angle?

If you're talking about the right angle between vertical and horizontal, the answer is that you don't. Extend the trench a few feet to the side of the sidewalk so that you can push the pipe in horizontally.