I put my own together.  Just the basics:
bandaids- for minor cuts
moleskin/some other form of blister treatment- very important b/c if you’re doing alot of walking you don’t want a few pesky blisters to slow you down
Immodium AD/ Rolaids or some form of upset stomach relief – there’s not much chance of contracting a food borne illness but sometimes the stress of travel or the liberal use of olive oil (as is the case in most Italian cuisine) can really wreck havok on your digestive system
Pain Reliever- you will probably do more walking than you are used to and may need to use some sort of pain releiver for sore back or legs
Vitamins- more preventative than anything
Prescription medicine

Center for Disease Control says take a general antibiotic in your first aid kit. You might check their web site or give them a call to see what they might suggest. If you’re not used to flying, you might get a sleep aid.

Cipro is the most common Abx prescribed. It can be used for traveler’s diarrhea.
But I would recommend to use in serious case, to avoid resistance, as most cases are self limited…
 
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i think its not a big issue in Europe – you are likely to be in the vicinity of qualified medical aid at all times. new to backpacking sounds like you’re not going off the beaten track.
 
having a jungle trip in the Amazon would be another thing.
 
Just the basic stuff – plasters / bandaids, pain killers, anticeptic cream, diarrhoea remedies (even that not very likely to get used)

You can get cipro (or other antib’s) over-the-counter many places in europe. No stress there. Depending on where you go though (no need in W Europe), antibacterial handwash (the kind you don’t rinse off) is handy. 

I second the votes for pain reliever, immodium (anti-diarrheal capsules), and band-aids (plasters). That’s all I take, and I take just enough to get me through a weekend of possibly not being able to get to a pharmacy or supermarket—because not all countries have pain reliever and anti-diarrheal available outside of pharmacies. If I were prone to blisters, I’d also bring some powder or moleskin. 10 Advil, 10 Immodium, 10 plasters various sizes in 1 small ziploc snack bag = not much weight or inconvenience to carry, but a godsend if something does come up (pun intended).

I know that REI has several backpackers first aid kits, although it’s really easy to make your own, too. I usually take a little more than some people here. I packed aspirin, immodium, alcohol wipes, band-aids, tweezers (for splinters and such), kleenex, sudafed, over the counter sleep aid, Airborne/Emergen-C, fingernail clippers, antibacterial gel, a thermometer, q-tips, and wet wipes. I didn’t use most of it, but it was nice to have. Like others have said, most of the stuff will be readily available in Europe, but you don’t necessarily want to spend the time to go looking for it when you’re not feeling so great.
 
I found it helpful to have the Airborne along because I think that taking it a couple days where I felt like I was coming down with a cold prevented me from getting a full-blown cold. Could just be my imagination though.

I don’t bother with a standard medical kit, just bring the essentials:
 
roofies – in the odd chance you find a European stunner who refuses your advances
 
vicodin – makes those naseauting art museums much more enjoyable
 
viagara – why waste your days wandering the dirty streets in Amsterdam when you could be shagging Thai prostitutes by the dozen hours on end
 
advil – for the morning after