Read about the herbal supplement you are considering taking. Read and gather information from many different sources, not just one.
Read about the reason you are taking the supplement. For example a headache, muscle soreness, etc. Read about other recommended supplements that can aid in your issue.
Make sure you are getting your information from a credible source. If it is a website that is trying to sell you the supplement you are considering, they probably will make it sound better than it is. Look for a source that is non-profit organization or look for unbiased research.
Consider the quality of the supplement. Since there are no regulations in the U.S. on herbal supplements you must make sure you are getting them from a reputable supplier.
Cheaper is not always better. Good quality supplements are going to cost what they are worth. This is also where research is going to help. Maybe you will find out that one brand is cheaper because it isn't fresh like the more expensive bottle. Sometimes the supplement may lose some of its effect by being a few weeks old.
You should always consult your doctor or medical professional before beginning any supplement regimen. Good luck and protect your health.