I would recommend checking out an app called TrueStock (https://apps.shopify.com/truestock), simple to install and use. It will predict how much sales you will have and provide inventory recommendations based from it. All you have to do is just click install and register, everything else is automatic.
Hope this helps!
What specific features are you looking for in an inventory management app? There are a few different solutions available but each have pros and cons depending what features you are after like PO's, product management etc.