To generate a point, you plug in a number for x to get the corresponding y value.
If x = 0 for instance, then the y value is...
y = x^2 - 4
y = 0^2 - 4 ... x is replaced with 0
y = 0 - 4
y = -4
So x = 0 and y = -4 pair up to get the point (0,-4). This is the y intercept as the parabola crosses the y axis here. It turns out that this is also the vertex point as it is the lowest point on the parabola.
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If x = 1, then,
y = x^2 - 4
y = 1^2 - 4
y = 1 - 4
y = -3
meaning (x,y) = (1,-3) is another point on this line.
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Repeat for x = 2
y = x^2 - 4
y = 2^2 - 4
y = 4-4
y = 0
Since we got a y output of 0, we have found an x intercept located at (2,0). The other x intercept is (-2,0).
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The idea is to generate as many points as possible. Plot all of the points on the same xy coordinate grid. Then draw a curve through those points the best you can. You should get what you see in the diagram below. I used GeoGebra to make the graph. Desmos is another handy tool I recommend.
Note: the more points you generate, the more accurate the graph will be
