Draw the triangle on a piece of paper with the base or longest side at the bottom and the tip pointing up so that your triangle will look like an arrow pointing up. It doesn't matter if any of the sides are bigger than the others or if they are not equal so long as it forms a triangle.
Get the measurement of the length of the bottom side or base. The length is the measurement of the bottom side from end to end. Write it down as the base or "B"
Get the measurement of the length from the tip or highest point of the triangle all the way down to the bottom or base line. You can do this by drawing a straight vertical line from the tip of the triangle down to the bottom line. Write the figure on your paper and mark it as "H" for height.
Finally, compute the area by multiplying .5 by the base or "B" then by the height or "H." The resulting figure will be the area of the triangle in square inches or square feet whichever unit of measurement you used. Example: .5 x 10 inches (Base) x 5 inches (height) = 25 square inches