If you have a just begun training with weights then you will find that your forearms grow without you even having to do forearm workouts directly. This is because the forearm muscles are involved in gripping, so the heavier the weights you grip, the bigger and stronger the muscles in your forearms are forced to become.
Gradually as your training progresses you will reach a point where you will find direct forearm training beneficial. When you reach this point I recommend you do barbell wrist curls for your forearm workouts.
Sit on the bench with your legs over one side feet firmly on the floor. Hold a barbell with a grip several inches wide with an underhand grip. Move the barbell into a position where your forearms are laid across the top of your thighs and your hands are hanging over the edge of your knees. Allow your wrist to bend all the way backwards and then using your wrist, curl the barbell up as high as you can go.
You will need to work the other side of the forearm as well and you do so by repeating the same exercise only this time using an overhand grip.
I believe you will find direct forearm workouts to be beneficial eventually because as the weights get very heavy your grip may start to fail and this is a good way to make the grip stronger.