Preacher curls can be good, but please start lightly. Begin with a weight you KNOW you can easily do.
Broadly:
1. It's a key exercise to strengthen your arms.
2. Preacher curls do a great job of hitting the lower portion of the biceps muscle.
3. Remember to use less weight than you would with a regular biceps curl.
4. If you don't ego lift, you will probably never have to worry about an injury.
5. Make sure the whole back of the arm is supported by the pad, and train throughout the full range of motion.
Again, you don't HAVE to use preacher curls to train your arms. Plenty of lifters go through their entire lives without these exercises. Feel free to consult your doctor or your trainer before doing these, or skip them entirely.
Have a magnificent day!