How to know if you need new brakes?

I have a 2012 Honda Civic I bought used in May 2015 with 45 thousand miles. I have 51 thousand miles now and my brakes are stopping but I notice I really have to press my foot on the pedal to brake. I have no idea if its the original brakes either. Also no kind of brake/ warning light has come on. If I need to replace them about how much will it cost?

Only way to know is to have them checked.

Changing your front brake pads should be $100 or less. But hat isn't going to make it less effort to stop. You have a problem with your Brake Booster. If it's Vacuum powered you may just have a Vacuum Leak. Have a friend or mechanic check your vacuum lines and fittings. You can also listen for a whistling sound when it's running.

Have a mechanic look at it. Probably just needs to be bled

If you can do it yourself(assuming you already have a good set of tools and a jack/chucks) you can do a complete brake job the right way using parts from Autozone or Advanced Auto Parts(rotors and ceramic pads) for roughly $230 more or less. It will run anywhere from $300 to $600 for a shop to do it. Lots of shops who will do it for cheap will skip out on lubing the slider pins, which could end up seizing and cause the pads to wear irregularly.