You certainly can go it alone and create your own system - ideally by adapting an existing standard, rather than building an entire taxonomy and ontology from scratch.
If you're certain that your specific needs cannot be met by adapting an existing metadata standard you can find some excellent guidance from the following resources:
Guide to Writing "readme" style metadata: From Cornell University, Research Data Management Service Group
Creating a Metadata Schema: Where to Start: From ISO (International Organization for Standardization)