2 simple photoshop questions

I have a design that has 3 different colors, but the colors, when you zoom in, are actually several different shades of whatever color I am zoomed in on. For example, when I am zoomed out the green part of the design looks like one shade of green, but when I zoom in very close I can see that it’s many different pixels of different green shades. Is there a tool where I can instruct Photoshop to transform all of my shades of green into one shade of green that I want all of the green pixels in the design to be? And can I do the same for the red and the blue? So by the end I do truly have just 3 solid colors in my design?

My second issue: I want to make all of the white in the design transparent. The white right now is the background and some mixed in the design. If i eventually want to use the design on a t shirt, right now it would have a white background, etc. How can I make it so it’s transparent so it’s just the design and no white background?

As you might be able to tell I am a Microsoft Paint user lol and my dream is to eventually learn how to use Photoshop. Can anyone help me with these two things? thank you all