Still life objects such as flowers can look great when simply lit by sunlight from behind. However, if you're working indoors next to a window they often look out onto gardens, streets and other distracting objects which don't make great backgrounds and can spoil the shot. You can, of course, remove the background with editing software but by simply placing a plain object behind your subject you'll save yourself a lot of time. A simple piece of card or cloth will work just fine.
You have to be careful where you place your new background as it can block the sun but by holding it or, if you can, getting someone else to hold it, the background can be moved around while you look through the viewfinder to see what position works the best. The trick is to move it as high up behind the object without any sun being shielded. Don't position the object you're photographing too close to the background either as no light will be able to get to it. If you enjoy doing these sort of photos you could even create a purpose built set up that could be used again and again for indoor still life shots.
If you have to position your camera/background so some of the window creeps in at the top don't worry; you can crop it out later when you get your image onto the computer.
When holding the flower take care with your shutter speed as going too slow will result in shake and as you're working with direct light, going too slow will leave you with a shot that's over-exposed. Don't use a too wider aperture as the petals towards the front and back of the image will start to lose focus. Try starting with f/8 and reduce/increase from there.