assuming that you have got the required licenses for AWR and Tuning Advisor (Diagnostic Pack resp. Tuning Pack) you can start reading the documentation:
2 Day + Performance Tuning Guide: https://docs.oracle.com/en/database/oracle/oracle-database/12.2/tdppt/index.html
The documents link is for general database tuning, and not specific for EM Express
Can you share me please the one for EM Express? especially the performance hub?
Can I have a fast track learning as well?
Is it possible to get a 1 week database performance snapshot that shows which day is the busiest in workloads and which hour of the day as well?