Sometimes you can find really useful second hand stuff for cosplay in apps like vinted, and they are cheaper too
Also, petticoats are very good for making cosplay skirts fluffier, you cand find them in plenty of colors and lengths.
When it comes to going to a convention, I recommend that you bring a bag with some of the things that you might need, like water, makeup or a little sewing kit.
And I DEEPLY recommend wearing comfortable shoes to conventions, you're going to walk a lot, high heels are not a good idea
Those are some of my tips :D