Find a picture of a park bench to work from. Although you've probably seen many park benches in your life, it's easier to draw from a picture if you can find one that fits your needs. Select a picture that is large enough to easily make out the details. If you are having a hard time finding a picture of a park bench that fits your needs, try looking online at a photograph database like morguefile.com.
Sharpen your pencil.
Draw the outline of the park bench. You may do this using a ruler for the straight edges, or free hand. Most park benches are made of slats of wood. Do not worry about drawing each individual piece of wood at this point; just draw the overall mass of the chair. You may draw lightly at this point and then draw over your lines more darkly when you are sure that you've drawn it to your standards.
Draw the outline of the legs of the park bench. Again, you may feel more comfortable drawing lightly and then drawing over your lines with darker lines when you are sure of the placement.
Start drawing the more obvious details. Fill in the body of the park bench with the lines for each plank. Use the ruler here if you feel it helps.
Draw the finer details like the bolts that hold the planks on the bench (mounds of metal represented by circles, usually) and grass beneath the bench (usually represented by short lines made with the pencil).
Shade the park bench where appropriate. There will likely be a shadow beneath the bench on the opposite side of the sun.