If you have never lived in a so called 3rd world country, WELCOME. I left the U.S. over 6 years ago and landed in Medellin. After 2 and a half years I moved to Bucaramanga and then after another 2.5 years moved to Barranquilla. I should have never left Medellin. Hands down the best by far. Friendliness of the people, the climate and accessibility to a more modern life.
TIPS -
VISA - The VISA process can be a hassle and without using an Agency in Medellin I probably would have left the Country. By the time you pay for a plane ticket to Bogota, hotel, taxis, food, etc.. it's cheaper to pay an Agency. Unless you just want to visit Bogota, fight the traffic, and stand in long lines.
THE LITTLE THINGS WE LOVE - If you like your American foods, you now have that problem solved. PRICESMART is a Costco or Sams Club copy that is loaded with pretty much everything you could want from      the  U.S.  and has expanded in 2014 throughout Colombia.
MAIL - Get a mailbox in Miami at TCC and have them forward you mail and packages to you door FAST and at a very reasonable price. Â
BANKING - Keep a Bank acct in the States. The ATM's here accept your cards and the Dollar is awesome at this time. In the past, years ago I was getting 2500 pesos on the dollar. Then there was the 1600 pesos per dollar which was terrible. Now it's doing great again.
DOCUMENTS- Have a copy of any documents you might think necessary. Birth Cert. Marriage or Divorce papers and a copy of your will if you have one. Carry copies of your ID's when out and about. Never the original..
Take the time to get accustomed to things getting done in slow motion. Nothing happens all that fast here.. The #1 used word in Colombia is MANANA, and sometimes manana can take weeks.
SECURITY- The big Cities here in Colombia are more or less the same as BIG Cities around the World. They are good areas and there are bad neighborhoods. ASK. It's simple. Don't wear your gold jewelry when walking around town. I sold mine after a year here.  NEVER walk off a curb without looking in every direction 3 times. I'm serious. Pedestrians have absolutely NO RIGHTS here. It's every man or woman for themselves.  If for some reason you need to call the police, buy a sandwich. It could be a long time before they show up. Just be aware of your surroundings at all times.
I remember when I first visited Colombia about 8 years ago, "Colombia, the only risk is wanting to stay".. It was true, I love it here. Just be yourself, be friendly, and respectful of others.Â
GOOD LUCK.............