First you need to get your line in slope-intercept form (y=mx+b)
So....
3x-y=10 (Right now it's in standard form)
3x+-y=10 (Change minus to +-)
-3x -3x (Subtract 3x from each side to get your "y" alone)
-y = -3x + 10
y = 3x + -10 Multiply each side by -1 and..... Now it's in slope-intercept form!
Your slope (m) is 3 (you'll find it right in front of the x - it's the coefficient of x)
Your "perpendicular" slope is the opposite reciprocal (-1/3)
If you need an equation, just plug the new slope into your equation y = (-1/3)x + -10
**There's a really cool shortcut for this, but first you need to know how to put it in slope-intercept form to find your slope :)