Curtains are so hard to choose! Your decor is quite sleek and modern with just a hint of fancy, so probably you should go for either plain or a modern, understated print. John Lewis is good for ready made curtains, as is Dunelm.
Decide whether you need blackout lining
Decide whether you want a swish rail or a round pole
Check the standard lengths online and measure up from about 5 cm off the floor to that point (I think JL longest is 228cm) and then fit your pole or rail. If a pole, measure to the bottom of the curtain rings; if a rail, measure to the top of the white strip.
Measure the width of the window and choose the width from the standard range - you want an extra 15-30cm on each side of the window to give room for pulling back the curtains without losing too much light, so for a wide-ish window you should choose the option that’s 50-60cm wider than your window, or for a narrow one, go for the option that is 30-40cm wider.
Think about fabric texture as much as pattern/colour. I’d keep it quite simple in your room.
Iron the curtains flat before pulling the ruflette tape, or use a steamer once they’re up.