you would graph this the same way you would any other y = mx + b equation, only, in this case, there isn't a y-intercept present in the equation. it's just y = mx.
y = 1/2x -- what you do is plug in x-values, then get the y-value and graph the point. choose easy numbers so the math won't cause you too much trouble (numbers like 1, 2, -1, and -2 are usually easy enough).
when x = 1...
y = (1/2)(1)
y = 1/2
(1, 1/2)
when x = -1...
y = (1/2)(-1)
y = -1/2
(-1, -1/2)
when x = 2...
y = (1/2)(2)
y = 1
(2, 1)
when x = -2...
y = (1/2)(-2)
y = -1
(2, -1)
that should be enough to sketch a graph. if you need more points, just follow the same process and plug more x-values in to get your y-values.