To find the equation of any line you need two pieces of information:
(i) slope and y-intercept (m and b) or
(ii) slope and a point (m and (x1,y1)) or
(iii) two points ((x1,y1) and (x2, y2))
In your case, slope and a point seems to be the way to go since you already have the point. So how to find the slope you need?
Find the slope of the line whose equation you have been given. Since the two lines are perpendicular, and perpendicular lines have negative reciprocal slopes, you can use this slope to determine the slope you need. For example, if the slope of the line given to you is 2/3, the slope of the perpendicular line would have to be -3/2. In other words, invert the slope and change its sign.
Now you have slope (m) and a point (x1,y1) and can use the formula y-y1=m(x-x1) to determine the equation of the line.
Hope this helps.