Hey, I travelled around alone in Colombia and I did not have any problems.
Have always your ID with you in case that the police stops and checks you (don´t take drugs with you) and be aware of pickpockets like you do everywhere else in Latin America as well but don´t be too afraid.
And if you are insecure you can always ask the locals (people that work in hostels/hotels) which areas are save, which aren´t etc.
In Medellín there are save taxis outside the airport. You shouldn´t have any problems. They also have a taximeter in Medellín and Bogotá. If you go to Cartagena you should always ask for the price before you leave! Ask in your accommodation for the right price. For example in the city center you always pay 5.000 PEsos to go anywhere. In the night it´s 6.000. If you go further, could be 7.000 or 8.000.