Look at this way. You need 500 words.
100 for your introduction
3 paragraphs of 100 each for your body/main part of the essay.
100 for your conclusion.
If you think of it like that, it will be easier to come up to the 500 mark. Just make sure that you have perhaps three or four good main points and you can end up with 3/4 main paragraphs. If you end up with four main paragraphs then you'll only need 50 words for your intro and conclusion. Hope this helps you for future essays. If you need any more help or some clarification just PM me😊