New, need some advice?

Hey there. i need some help figuring out how many calories I should eat to lose weight.
I'm about 150 lbs now and I hope to be about 125 by my 21st bday in mid-February.

I'm following the In Place of a Road Map plan because I did this in the past and lost some weight, but then when I started working full-time, and I wasn't as able to keep track if my calories and work out as often, my weight loss stalled and I haven't lost any in a few months.

I am able to work out more often now, and keep track of my calories easily so I'm really ready to get back at this. Unfortunately I have no idea how many calories I should eat.

I'll be working out 3 days a week, for 30 minutes, doing strength training. My job also keeps me really active, as I work in fast food. I'm on my feet all day long, for about 6 hours a day, running around, going up and down stairs, etc. Outside of work and my work outs though, I sit a lot, doing school, etc.

I used a calculator at Scoobysworkshop and put my activity level down as lightly active and it recommends I eat about 1700 - 1800 calories, but that sounds like how much I have probably been eating anyways, though on some days I'm sure I only ate about....1400 or so, to make up for eating too much the day beforehand.

So, any advice?