Well you have a fairly easy cosplay! ^^
I suggest going to your local fabric store like Joanne's or something of that kind near you, look for strapless dress patterns.
If I'm looking at the picture correctly, she's wearing a shirt and then a black skirt...
So, if you get a strapless dress pattern and then alter it to end around your hips or thighs (gives enough room to tuck it in, since it looks like it's tucked in) and then make a skirt, which is fairly easy, and you can find that pattern also there or just alter the lower half of you're strapless dress pattern for that as well.
I don't know what exactly her arm guards are, but it looks to be just like arm warmers, and you can find tutorials on how to make those, or find them in a local store =]
For material... I would just suggest that any type of material that you find at your fabric store that feels good and comfortable to wear, and easy to sew. There's really no specific fabric that you should choose from, it just depends on how it feels and if it's comfortable, and most importantly, is it easy to work with! Lol
I hope I helped at least some lol ^^
Good luck! <3