If you can't afford flying, taking a bus such as Omin Lineas or Plataforma10 is next best option. You can catch either of these buses from Buenos Aires and travel right to Iguazu Falls. The trip takes around 15 hours and you get to see nice scenery on the way, unless you travel at night. The night buses are cheaper though.
A ride on a luxury bus will cost between $100 (~1600 pesos) and $130 round trip depending on whether you choose a full-bed. If you fly, you will need to budget for transportation from the airport to Puerto Iguazu town. Taxis charge $50 one way. then, arrange for tours of Iguazu falls
Buses from Buenos Aires depart regularly from Retiro station and the trip takes closer to 20 hours. If you are pushed for time then it's best you fly as the flight time is around two hours. Once you get there, take your time at the visitor center to plan your visit. You don't really need a guide, though english-speaking guides are available.
By bus is a nice option. Take it from Retiro and will last around 13 hours. Is better to buy it from Plataforma 10.