As with any resistance exercise, you should use a weight that you can lift safely but which also challenges you. When starting out, use a light weight and see how you feel when performing the desired number of reps and sets. If it's too easy, increase the weight. Keep doing this until you find a weight that you can lift safely and complete all the reps, but which is a challenge toward the end of each set. You should feel a stretch in your biceps when lifting and lowering the weight. Then, as you continue to perform preacher curls regularly, gradually increase the weight as your strength builds.