Just cant get my head around it! help needed plz

Hello ppl. Please help. I just am lost when it comes to figuring out how many calories i need to eat to lose weight etc. Sites indicate cals consumed then minus burnt etc. Maybe i need to resit my maths exam coz i just dont get it. Please someone explain the formula. Im currently eating 1100 -1200 cals per days and averaging 600 cals per day burnt through cardio exercises. Im female, 37 yrs, desk job, 177lbs. I would like to lose 3 stone (42lbs) by end of may. Am i doing the right thing? What sort of calirie defecit does that give me. MFP states that based on my profile I should naturally burn 1800 cals doing nithing....is that the BMB?

Hope someone out therr can educate me.....!