How many calories should I be eating?

According to MFP, I should be eating 1200 calories (net) per day. My BMR is 1277 calories a day. I've been sticking to this since I started MFP and it's all been fine and good and I've been losing around 1 lb a week.

Until a couple of weeks ago I was doing mostly cardio with a bit of strength training thrown in couple of times a week. My goals have changed slightly since I joined the site and I now want to build muscle and sculpt my body rather than just lose weight. I now have a personal trainer that I go to and do more strength training per week than cardio.

I've heard a lot of things being said about needing to eat surplus calories to build muscle, but as I still have a bit of fat to lose should I still eat surplus calories? I'm worried that if I start eating too many calories I will just gain more fat and I'll never have any muscle! How many calories should I be eating? Help!

(BTW I'm 24, 5'3", am small framed and weigh around 123 lb at the moment)