Ask HN: How can I make a constantly-updating hotlist like HN?

I'm wanting to make a web app where there is a hotlist that will display trending topics first, kind of like how HN works.

I am using MongoDB for this and the data that effects each item is VIEWS, REVIEWS, and LIKES.

How can I calculate by how many likes, views, and reviews an item is getting to test if they are trending or not?