Figure out as best you can your household Modified Gross Income (MAGI) is likely to be for 2014. For that you can use the Consumer Reports cheat sheet that describes exactly how this works. If self-employed, at least a rough idea of what you expect to earn in 2014.
Go to the Kaiser Family Foundation’s subsidy calculator. Fill it out using the MAGI number you calculated. The subsidy it shows you, if any, is given on an annual basis. Divide by 12 to find what it comes to every month.