Curtains come in a variety of fabrics. In addition to considering the overall look, you want your windows to look like, keep in mind the amount of light that will pass through. Heavier, opaque fabrics block light better than lighter, more transparent fabrics.
If you want a lot of light through, choose a sheer or cotton fabric; if you want a moderate amount of light, choose a linen curtain, which offers a textured weave that lets some light in; if you want all or a lot of light Partially shaded, opt for blackout curtains, which have a special lining that completely blocks sunlight.
Within each fabric option, you can also choose from colors and patterns. Go bold if you want the curtains to be a standout feature in the room, or keep it simple if you don't want them to distract from other focal points in the space.