While I only made a fairly quick trip to Europe this summer, I learned a few of the ins and outs of how to make smart decisions while traveling. I was able to visit Italy and Amsterdam, while my little sister was in Europe for three weeks and was able to go to Amsterdam, France, Monaco, Spain, and Italy with her friends. Between our shared experiences, I have come up with a few things that every traveler should know (unfortunately, we learned them the hard way).
If you want the full experience of visiting a new place and fully immersing yourself in the culture, stay in hostels. As long as you're okay with possibly sharing a room with a few other people, they truly are the best way to go. Not only are they fairly inexpensive in the realm of things, they're also the best way to learn where you should go to eat, go out at night, and sightsee in that specific place (without having to rely on a guidebook). Locals usually run them, therefore they have the most authentic advice to give travelers. Plus, when you're sharing a room with other people, you get to meet some amazing people (bonus points if you stay in a coed room).
Do not, I repeat, DO NOT, ever travel by taxi unless there is absolutely no alternative. Especially in Europe, your best bet is to go by train or bus (inexpensive and quick), or to walk if your destination is close enough. Taxi companies LOVE to exploit travelers, especially young ones, and make them pay unreasonable amounts for even the shortest of trips. For example, my sister and her friends mistakingly thought taking a taxi from the train station to the hostel would be a good idea since it was about 15 miles away, and it turned out to be €70 (almost $80), when they could have taken a train for about €12. If you do have to take a taxi, see if they offer a fixed rate to your destination, or read the rules the country has set. Some places set a maximum cost that the taxi can charge you on one trip from an airport/train station to a hostel/hotel.
Make sure your credit card (most places only take Visa or MasterCard) has a chip in it. Additionally, always carry cash with you. Most places overseas do not take credit cards, and if they do, they usually only take one with a chip in it. Your best bet is to bring cash with you wherever you go. If you need to take out more cash, I suggest going to an ATM because they usually have one of the lowest (or most accurate) exchange rate.
On your iPhone, Google Maps does not need cellular data or internet to show you your location. In times of crisis, don't be afraid to pull out your phone and look at your location. Although you cannot map directions to your destination, it will show you where you are and the street names around you, which was one of the most helpful things I learned. Your best bet is to carry a map with you too just in case, but for the most part Google Maps will do the trick. When in doubt, don't be afraid to ask for help, too! Locals are usually very friendly and helpful to people who are lost.
Finally, take lots of pictures, but don't be afraid to put down your phone and just enjoy yourself. Odds are your camera is not going to perfectly capture that moment, but your memory will.