Counseling in retail in not too hard in my opinion. If you can read a medication label (the one your computer prints out for the customer), you can counsel.
WHEN I have to counsel, I tell them basics: Name of the drug, what it is, what it's for, how many times a day to take it, and for how long. Mention a couple common side effects and how to take care of it, and that's all. Then any other questions they have, I tell them to call me (24 hour store) or call their doctor. Many times they do call me back when we are not busy and we can having a more detailed conversation if needed.
For example,
"Mr. Smith, This is your prescription for amoxicillin. It's an antibiotic used to treat infections. Your going to be taking 1 capsule 3 times a day. For the first dose, I want you to take 2, that way the medicine is going to kick in faster. Take it for 10 days, and make sure you finish it all, even if you feel better. Some people may experience some stomach upset with this medication, so if you do so, you can take it with a little bit of food. Any questions you have for me, you can always call me here or ask your doctor."
You see how easy it can be? Everything you mention comes directly from the label your computer prints out.
I don't really ever do more than that, and mentioning serious side effects or allergic reactions seems counter productive to me. If a patient sees their arms swell up or break out in hives or have trouble breathing, they know to call their doctor or go to the ER. I don't mention these things unless the customer asks because then it may scare them, prevent them from taking the drug, or whatever.
If you want a good drug reference, LexiComp for iPhone is so f'n amazing. You can type in the drug name in like 2 seconds, go to "Administration" section and it tells you how to take it (with or without food, what you can mix it with, etc), and look up "Adverse Reactions" section and it states right there what the most common side effects are.
Good luck!