You can just plot small vectors at a rectangular grid of numbers in the x-y plane say from -5 to 5, the slope of the vectors of course is the value at each point so for example at the point (1,1), I'd draw a small line with a slope of 0. At the point (0,2), the line would have a slope of 2 and so forth. Once you understand the meaning of that, then try StreamPlot in Mathematica which plots the vector field of {x'(t), y'(t)} so if I let x=t then I'd need to plot the field {1,y-t^2} like so:

StreamPlot[{1, y - t^2}, {t, -5, 5}, {y, -5, 5}] and that gives me the phase portrait below which I've also drawn a solution (in red) to the DE for y(0)=1.

Also check out "Differential Equations" by Blanchard, Devaney, and Hall. It's the best reference for these types of problems.