The trick to these problems is to note that, unless it's a simple application of basic geometric formulae, they will always give you two pieces of information, such as a statement about perimeter and then a question about area. Then you need to write the two equations related to these two pieces of information, solve one of the equations for one of the variables, and then plug this into the other equation. Find the dimensions of the rectangle.
Some problems are just straightforward applications of basic geometric formulae.
Suppose a water tank in the shape of a right circular cylinder is thirty feet long and eight feet in diameter. What they are asking for here is the surface area of the water tank. If the diameter is eight feet, then the radius is four feet. Always remember to put the correct units on your answer. Since the dimensions were given in terms of feet, then the area is in terms of square feet.