Hi Molly,
Most DIY stores will be able to sell you plasterboard fixings. These are suitable for attaching light objects such as curtain rails. They consist of small, plastic fittings which screw easily into the plasterboard and are sufficient to allow the rail fixings to be screwed directly into them. For extra security, if the curtains are particularly heavy, a dab of glue between the rail fixings and the wall should do the trick. Hope this helps. Good luck with your new flat.