What if I want to define a resource but the data I have does not match all the predefined fields?

I am new to FHIR and I am trying to understand how the resources work.

Assume I have patient data and I want to store it as a Patient resource define by FHIR.

Patient Resource: http://build.fhir.org/patient.html

But say the data I have is limited to Patient Name, Gender and Birthdate. Do I just omit the other fields while defining my Patient Resource?

check the cardinality on the fields; if the cardinality is 0..*, then you can omit them, yes. In the case of patient, nothing is 1..n, so you can omit whatever you want