I want to convert my image to solid black.
How can I do this in photoshop?
Here are two easy ways, but first, convert your image to grayscale (Image > Mode > Grayscale).
In each case what you're trying to achieve is increased contrast, i.e. fewer gray levels between white and black, while preserving a few gray levels between black and white (see the gradual transition from black to white on the back of the subject's neck in your second picture.)
Levels — Image > Adjustments > Levels
In the Levels dialog, everything left of the black point on the left becomes black, and everything to the right of the white point becomes white. When you move then closer together, contrast increases. Moving the gray slider increases or decreases contrast between midtones and blacks or whites.
Gradient Map — Image > Adjustments > Gradient Map
The gradient map is another interface for doing the same thing. After selecting the submenu, click on the gradient strip to edit the gradient. Set the left slider to black and the right slider to white by clicking on each one, then slide them together to increase contrast and control the black-to-white transition.
Make the photograph again, but not against a complex backdrop but with a flat one, like a flat wall, preferably white, or with a flat color, that is different from the skin tone or the clothes that the person is using, in the reference image what makes the black and white image stand out is the white sky behind it.
Then It would be easier to separate the person from the background. Once you separate it from the background then you can make it into black and white
If you absolutely need to work with the photo used in the question then you could use a color selection tool to select all of the green and make it white, then you need to carefully mask all of the other elements (trees and ground) and make them disappear as well, so that you are left only with the figure in the foreground. There is no automatic way to do this.
Once you separate foreground and background just turn it into black and white and increase the contrast using levels or curves.
You have to start by creating a mask of the person. You can do this with the Magic Wand selection tool. Once you have the user selected, you can create a mask from the selection. (If you want to spend a few dollars to make your life easier, you can buy Topaz ReMask to make this job easier.). Once the mask of the person is done, hit Ctrl-i to invert it.
Once you have a mask, you can independently process the user and the background. To get the effect you want, create either a Levels (simpler) or Curves (little more complex but more powerful) adjustment layer. Copy your mask to this layer. Adjust the sliders as described in @onacosmicscale's answer to get the effect you want. With the mask in place, your changes will only affect the area not masked, i.e. the person.
If you want the silhouette to be black and white, then you can use a Hue adjustment layer to lower or remove the colour as well. Again, copy your mask to this layer.