I need to convert 75 ohms balanced impedance to 50 ohms unbalanced impedance using a balun. How do I do that? I am new to this and don't really understand the concept of a:1 voltage and current baluns.
To clarify your confusion, some transformers have isolated primary and secondary windings. Some don't and have only one winding with a third tap-off point. These are cheaper to manufacture and can be used where isolation is not required. In power transformers they're often used to step the mains down say, from 230 V to 115 V or similar.
In your case you have a balanced signal which should feed into a balanced load. The balun provides such a load and the two halves of the balanced signal contribute to driving the unbalanced output. Notice that since you only have half the voltage on the output it will be able to deliver more current so its output impedance will be lower than the balanced signal.
I hope that helps.
A balun stands for "BALanced-UNbalanced" so you don't want a balun you want a transformer and, the ratio of primary to secondary turns would need to be the square root of 75/50 i.e. 1.225:1.
If you have a 50 ohm resistor on the lower turns winding it will look like 75 ohms from the perspective of the higher turns winding.