Change only small amounts at airports. Cash may be better than travelers checks in some countries. Check with official Tourist Offices at airports or city centers. You get city maps, information on hotels etc. Take the bus into town and also in town.
Some places have colectivos that advertsie the price on the windshield.
Taxis are OK in emergencies. Buses are good for long distances, too. Trains may be more comfortable but don't go to as many places. Hostels and backpackers places are cheap, and you meet many very interesting people from many parts of the world. For emergencies use MasterCard or Visa.