How do I build an Ontology from scratch ?

The easiest and most used way to build an ontology is to use the protege editor. You can either start with creating a new file or importing an already existing ontology and add more terms if you need to. It’s very unlikely that you are the first person ever that wanted to build an ontology for that topic you are considering. So it is highly recommended to search for an ontology similar to the one you want to build and inherit as much as possible from that. That is a general advice: Define as few as possible new by yourself. It is very likely that there is an ontology out there that has the class you just want to define. It is totally valid to build an Ontology with only imported terms and just assembling terms in a different way and adding axioms. That is called the open world principle stated by OBO Foundry. A good address to look for classes is the Ontology Lookup Service provided by EBI (https://www.ebi.ac.uk/ols/index). There is a good tutorial on how to work with Protege mentioned above in the section Protege.