Here’s a nice reference of Ariel from The Little Mermaid for your Disney costume. You’d need some sort of bikini top that resembles sea shells. As for the “mermaid tail”, you need something green with sequins to resemble the “scales”. You can either wear green tights or a long straight line skirt. Or to make it easier for yourself, check out this full on adult Disney costume. But remember, what makes Ariel stands out is her red hair.