Bangkok International airport transit?

Im travelling via Bangkok not too sure what the airport is called. I have a 12 hour stay at the airport, and wondering what to do. Does anyone know if it easy to leave the airport and head to the city? Do you need to get a visa for that day? How much is it? And who would you have to ask at the aiport regarding this? I have a british pport and was told I could leave and come back without any issues by my agent, althought he wasn't too sure.