A riddle is a type of poetry that describes something without actually naming what it is. It is a puzzle that you have to solve.
You have to pretend to be the thing you are describing so use I. You can start your sentences in different ways to make your poem interesting such as:
I look like
You find me
You should finsih your poem with the question 'What am I?'
Remember to start with the hardest clue first to keep the reader guessing.