Difference between database vs user vs schema
In Oracle, users and schemas are essentially the same thing. You can consider that a user is the account you use to connect to a database, and a schema is the set of objects (tables, views, etc.) that belong to that account.

See this post on Stack Overflow: difference between a User and a Schema in Oracle? for more details and extra links.

You create users with the create user statement. This also "creates" the schema (initially empty) - you cannot create a schema as such, it is tied to the user. Once the user is created, an administrator can grant privileges to the user, which will enable it to create tables, execute select queries, insert, and everything else.

The database is the thing that contains all the users you've created, and their data (and a bunch of predefined system users, tables, views, etc. that make the whole thing work). You should look at the Oracle Database Architecture documentation in the Concepts Guide (actually, that whole page is worth a read - there's a section about users and schemas higher up in that page) to get an introduction to what a database is, and what a database instance is - two important concepts.

You can create a database with the create database statement, once you've installed the Oracle software stack. But using dbca (database creation assistant) is easier to get started.