This is very important and can be very beneficial for beginners. It allows you to track your progress, see what exercises you enjoy, which ones you are improving the most, and what areas you need the most work on. Your journal should include the date, time of workout, duration of workout, the exercises, sets, reps, rest between sets, meals for the day, and supplements you may be taking.
One of the most misunderstood areas for new bodybuilders is nutrition. I talk to guys all the time that have no idea of their daily calorie intake, their daily protein intake, their carbohydrate intake. They have no idea of what types of foods they should be eating, or when they should be eating them. They don't know what supplements do what and what they should using. Let me refer readers to some of my other articles that detail these areas.