Two lines are perpendicular if they cross at right angles. You can test this by extending the lines until they cross (if they do not cross then they are parallel). Where the two lines cross, draw a right triangle, and then see whether the triangle conforms to the Pythagorean Theorem - if so, then your lines are crossing at a right angle, and they are perpendicular.
If Line A and Line B are both perpendicular to Line C, then Line A and Line B have to be parallel to each other.
To find out more about geometry, check out these books from Amazon.com or from your library: