Since Spanish is the official language of Colombia, it should be relatively easy to get around in the larger cities knowing basic Spanish. In rural areas, however, it is not uncommon to find villages that still speak indigenous languages, although the youth in these locations will more than likely speak Spanish as well.
People in Antioquia speak a very clean Spanish, with good pronunciation and sentence structure. Some slang is used, but for someone with a basic level of Spanish knowledge, the locals are easy to understand. Certain "Paisa" slang exists and while it's worth knowing it's not very important. Some keys would be "Parce" or "Parche" which means friend/pal/chum. Often will be converted into a verb like "parcheamos" which basically means, "hanging out (with friends)". "Pa" is also used often to replace "por" just as a short form, if someone were to say "pa'ca" its short for "por aca" meaning, here, similarly "pa'lla" is short for "por alla" meaning, over there.
Spanish classes of all sizes, prices, intensity levels are available across the city, again it's a quality Spanish that would serve you well in other Spanish speaking countries as well.
All international flights arrive at Rionegro's Aeropuerto Internacional Jose Maria Cordova (MDE). Regional flights depart from Aeropuerto Enrique Olaya Herrera (EOH) near El Poblado. Double check when buying your tickets, but the vast majority of flights depart MDE. International flights include multiple daily services from the USA (Fort Lauderdale, Miami, JFK) Lima, Quito, Panama City, Mexico City (starting March 2, 2015) and from as far as Madrid.
From MDE taxis can be had outside arrivals at the flat fare, the sticker in the windshield will show the price. Sometimes the driver will charge 65,000 sometimes 70,000. It appears to depend on the route he's taking or how far into the city you're going, either way it shouldn't be much more than that even if he gets lost. Alternatively there is a bus service to take you into downtown Medellin for under 10,000 pesos payable to the driver (or his assistant) in cash. It'll stop at a few points in between, including Terminal del Norte, one of the city's 2 major bus terminals and also a major shopping mall. Taxis can be had at either of these stops to shorten your trip or if going all the way downtown there are a bank of taxis at the location all day. Word of warning, the neighborhood (Centro) you'll arrive in is not the greatest at night. Many motels are around and the drop off point is immediately behind Hotel Nutibarra. This is a bustling area during the day, but if arriving late at night, it's not advisable for new visitors to arrive there without a game plan, ie: immediately getting in a taxi. The airport busses can be taken from downtown to Rionegro as well, price is the same, busses depart roughly every 15 minutes or faster if they fill up, depending on traffic the ride takes up to an hour or so, leave yourself time to get to the airport. The busses are obvious as they have pictures of airplanes and are labeled Aeropuerto JMC. City busses do not go to the international airport, feel free to ask the driver, but if you see a bus without "Aeropuerto JMC" and planes on it (usually the bus is green and a bit on the smaller side) then it'll be stopping at Enrique Olaya Herrera.
Larger city type busses are available outside JMC airport going to Rionegro if you happen to be staying there.
With the city's spring like climate, foot is often the favored way to get around. To cover longer distances taxis are ubiquitous and cheap, it's relatively safe to hail one anywhere on the street, but single female foreign travelers may prefer to call especially at night. Buses are frequent and will stop just about anywhere you ask or flag them down along the route. The placards on the windshield will indicate key points they'll pass, but only locals or regular users really will know how they'll get from each place to the next, they also pass by quickly so you may have to read a few before figuring out you missed the one you want, don't worry the next one isn't far behind, the fare must be paid in cash or with a slightly more complicated "integrado". Fares range from 1600-1800 COP, it seems to depend on the route. The Medellin metro system is world class and spotless, it does a great job of following the Rio Medellin north to south and east/west there's another line from the central station San Antonio. There's also 3 metrocables that extend into the barrios from Estacion San Javier and Estacion Acevedo (which leads to another metrocable to Parque Arvi from Estacion Santo Domingo Savio). Previously these barrios were very dangerous but feel free to visit them during the day and great get views of the city from well above. Metroplus lines are also available (similar to Bogota's Transmilenio) to supplement the metro.
Any areas unreachable by metro/metroplus/metrocable will have bus service, but it's often difficult to determine which bus is the correct one. When in doubt, take the metro to reach the stop nearest your destination then either get a taxi or ask someone around which bus to board.