You are alone. You have a senior, but if you call him asking for pain med orders, he/she will hate you. Just ask the nurse if the patient is allergic/sensitive to any meds. Then pull out your pharmacopeia. After a few times of this, you'll get the hand of MSO4 1mg IV q4hrs PRN pain. You'll likely become more familiar with Zofran 4mg IV q6hrs PRN N/V.
I am sure my colleague meant, "you are NOT alone"!.
You aren't. Everyone is scared at some times, and everyone was scared all the time when they started internship. You will soon come to see that most of the calls from the nurses are for minor things.
If it doesn't appear to be an emergency, you can ask them if you can call right back - whip out your pharmacopeia/whatever and find something suitable. You can ask the nurse what the patient has used before that has worked (often they know the answer to what they and the patient want, but don't want to tell you what to order). Don't always assume the patient wants or needs narcotics - while IV morphine is fine in most cases, some patients just want Tylenol or Motrin; others may benefit from Percocet, Vicodin or Tylenol with codeine (lasts longer than IV narcs).
There are also lots of pocket books out there to help you, as well as colleagues. Most common calls (which will vary somewhat from rotation to rotation) which may need medication:
Pain - as above assess need/want; what has worked before; patient's age and weight; many young doctors are worried about overdosing patients, you should be just as worried about underdosing and allowing the patient to have unrelieved pain. You will have a choice of everything from Tylenol, Motrin, Toradol, Tylenol with codeine, Darvocet, Percocet, Vicodin, Fentanyl, Morphine, Demerol, Diluadid, etc. oral/rectal/iv/im/patches/pca pump
Nausea - Zofran works well for many; some hospitals restrict it to post-op patients. Consider phenergan or compazine (can have extra-pyramidal or mental status changes with both), Tigan, or some good old crackers and warm soda.
Insomnia - Ambien is probably mostly used; benadryl for kiddies and some elderly (some will go crazy on it); tylenol pm if on your formulary
Tachycarda/Bradycardia or other rhythm disturbance: do not handle this by yourself; call your senior. If a patient is post op tachy with low blood pressure, likely due to volume depletion, but still call for assistance when you first start out. On IM the reasons for these problems can be much more complicated.
Etc. there are entire books written about this stuff. No one expects you to know it all, or not have to look it up. The nurses know and hopefully will be kind and helpful to you as you start out.