*Know what you don't know.
This is one of the most important pieces of advice I can give. It's fine to experiment a bit but only with safety guidelines in place. Before you put a herb in your mouth or on your body, do some research on it. Some companies such as Mountain Rose Herbs offer good information. However, don't rely solely on companies who have an interest in selling you the product to give you information about that product. Herb books written by practicing herbalists are probably the best sources but you can also use reputable websites such as Henriette's Herbal to do research.
*Know your allergies.
This is important whether it's a food or a herb. Be aware of your allergies and intolerances and how they might translate into the herbal world. For example, if you have an allergy to gluten, it's important to make sure your oatstraw isn't processed in the same facility as wheat. If you are allergic to aspirin, you should probably avoid willow bark, as willow bark contains salycilic acid which is similar to the chemical makeup of aspirin. Some, although not all, people who are allergic to ragweed may have problems with chamomile.
*Know your illness and tendencies towards illness.
If you have a chronic disease, it's good to have a general idea of how it works. For example, if you have an autoimmune disorder, certain herbs which strongly stimulate the immune system may not be appropriate for you. Also, be aware of any tendencies towards illness in your family and make yourself familiar with herbs which are traditionally used for those illnesses, as well as any lifestyle changes which might be helpful. You can also build your herbal "medicine" cabinet with herbs which might come in handy for your family's needs. For example, as the mother in a family with active children, I make sure to keep herbal preparations for cuts and sore muscles on hand.
*Know your medications.
If you take medication, whether prescription or over the counter, it's a good idea to make sure herbs aren't going to interact with your medication. Some food-type herbs such as spearmint taken in moderate amounts as a tea are probably not going to react but there are other herbs which are more likely to interact with medication and it's important to be aware of this. If you take prescriptions regularly, a good place to start is your research is Prescription for Herbal Healing by Phyllis Balch.
*Know your herbal allies.
Pick five versatile herbs, ideally ones which grow locally or which you can grow yourself, and get to know them well. Try herbs out on yourself before you get ill. That way you are aware of the way they interact with your system before you need to call on them in a crisis. If you are particularly sensitive and prone to allergies, you can start by rubbing a small amount of the herbal preparation on your wrist and if you have no reaction to that, you can take a tablespoon internally (provided it is a herb and preparation appropriate for internal consumption) and proceed from there to a larger amount such as a cup. If you aren't particularly sensitive, you can start with a cup of herbal tea and observe your reactions. Keep the herbs with which you are familiar on hand so you can use them when you need them. The flip side of the coin of respect for herbs, is you also come to trust those you know and have used many times. With respect and trust you can go far.