As a former personal trainer...

I recommend getting to a gym and having a Resting Metabolic Rate test done. Most places have some way of doing this - usually, you sit and breathe into a little gadget for awhile, and it spits out fairly accurate information about how many calories your body will burn if you don't move a muscle all day. If you drop below this number of calories (either by eating too few or burning too many), your body will start shutting down into starvation mode. There some formula that I don't remember for increasing this number by certain percentages based on the rigor (or lack thereof) of your normal daily activity, and then you can add in the number of calories burned for any workouts you plan to do. This'll give you a fairly accurate picture of how many calories you're actually burning in a day. The key to losing weight is to consume fewer calories than you burn through your physical actions, but NOT less than the number of calories you need for basic bodily functions.

If you don't have access to (or the money for) a consultation with a personal trainer to determine this accurately, there are plenty of websites that offer ways to estimate RMR and calories burned during certain types of exercise. They're not 100% accurate, but they'll give you a decent idea of the appropriate range.