Welcome to EPZ and the junior gallery Charlotte.
Its hard to believe this is your first picture, and overall, its not bad.
Theres a couple of tips and tweaks I have included in a mod Ive uploaded.
Her head is too low in the shot, - its a portrait of your sister, so get closer, and try to get the eyes approx 1/3 the way down the shot. Place her off centre, so she is about 1/3 the way across from one side.
When shooting against a bright background, such as here with the sky behind, use fill flash (check your manual) as it will brighten the subject which will otherwise tend to be a little too dark. Try to get her to have her eyes fully open, - the eyes are essential in a portrait.
Check the mod where Ive made these changes in Photoshop, cropping, brightening her face and hair, reducing saturation slightly, and see if you prefer this method of composition. Also, Ive sharpened the image, - you need to remember to apply sharpening after you re size for the web, as compression can cause the image to be too soft.
I hope you find this helpful, and learn lots from the site,