On my very first solo overseas trip, I spent three hours standing around a deserted airport in Bombay (it was still called Bombay, back then), waiting while my luggage sat, unattended, on the tarmack. On another trip, I plugged a borrowed laptop computer into the wall in Hyderabad and watched while the outlet popped and sizzled before I yanked the power cord out of the wall. On other trips over the years, I've ripped my luggage, misplaced a passport, lost my purse, been unable to change money (pre-Euro), had killer headaches, and been inappropriately dressed while sight-seeing -- all things that could have been prevented, if only I knew then what I know now.
But you can learn from my experience (and, of course, share your own in the comments!). Here are some other things I wish I’d known the first time I traveled abroad by myself.
Plug your computer, iPod, cell phone, etc. into the wall to charge it, but unplug it and rely on batter power when you use it. The less time it spends plugged into the wall, the less likely it will fry if there’s a power surge or other electrical problem.
Make at least two copies of your passport -- front, back, ID page, and any visas. Leave one copy at home, and zip the other copy into your suitcase. If something happens to your passport (or you get pickpocketed, or it goes through the wash) it’ll be much easier for you to get help if you have the important parts photocopied.
Change a little bit of money before you go. Most big banks will do this if the amount is small ($50 or so). It’s hard to tip a taxi driver with a traveler’s check, and some stores won’t take them if you’re only buying something small like a bottle of water. Also, depending on when you arrive, the currency counter at the airport might be closed, as well as the banks. And ATMs don’t always work when you need them to (I found that out that hard way in Belgium once). Not sure where to get foreign currency? There's your international airport, of course, but you can also try these options.
Bring a big scarf. You can use it as a light blanket on the plane, as a pillow on a bus, as a picnic blanket, as a head covering if you go to a cool old church, as a wrap at the beach, or as a shawl if you go out to dinner. A microfiber one can also be used as a towel or to dry laundry if you wash something in your sink.
You’ll probably have to wash something in your sink. Bring a length of clothesline (which you can also use with the big scarf to make a privacy screen, if you need to) and some packets of detergent -- you can find entire travel laundry kits online.
Duct tape fixes everything from ripped luggage to broken handles to holes in your clothes.
Bring your medications. That includes over-the-counter ones like ibuprofen. It's rotten -- rotten! -- to try to locate a pharmacy and buy some in a different language while coping with a raging headache. If you take a regular prescription, bring an extra month's supply as well as a copy of the script.
If you wear glasses or contact lenses, bring a copy of your prescription and, if possible, an extra set, just in case.
Compression bags (like Space Bags) are awesome. Use them on your way home so you have more space in your luggage for souvenirs (and so you don’t have to pay additional checked luggage fees).
What do you wish you'd known the first time you traveled solo?