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Few small pointers:
Orange: Neck is too elongated in proportion to her head. It' fine in proportion to her shoulders but her head should be dropped down a bit.
Red: Arms are far too straight. Go take a peek at other manga/anime drawings. You'll see that even when extended you can clearly make out a contour of elbow that is impossible to hide.
Green: Just a small note. Belly buttons are not that large and not a complete circle. Honestly, I always just put a typical comma shape (half crescent) in the direction my character is facing. That makes it a lot more believable.
Purple: Your girl's hips are a bit too compact in comparison to her gap. That's not physically possible. Now I know anime is not always 100% physically perfect but this is something that can really throw off the whole look of your character.
Hope I helped a bit! In the end, there are two BIG pieces of advice for any artist of any sort:
PRACTICE: Just keep going at it. Yeah it always sucks the first 1,000 times but at 1,001 you'll definitely see improvement from try 1.
Learn your target: Whether writing, drawing, sculpting, anything, you NEED to learn everything about what you are crafting. As the poster above said, learn anatomy. He's right. Go online and look up crazy amounts of others works. Try to match what they do with their body parts. It will help so much in the long run without sacrificing your unique style. In the end, there are very, very few artists who can claim they never spring-boarded off another's works at one time or another.
Good luck!