I want to to start my career as a database administrator. What should I learn? Should I learn SQL, MySQL, or mssql first?

MySQL runs on Windows and Linux, but is most widely used in Linux environments. Another commonly used SQL db is PostgreSQL.

Whatever you learn, you'll learn SQL. Note that some people misname Microsoft SQL Server as "SQL", but SQL is a standard query language supported by dozens of relational databases.

MySQL is the most popular open-source database, with open-source and enterprise payware versions maintained by Oracle, with "forks" maintained by other entities. MySQL runs on Windows and Linux, but is most widely used in Linux environments. Another commonly used SQL db is PostgreSQL.

Oracle itself is another SQL database, which is owned (shockingly enough) by Oracle.

Microsoft SQL Server is yet another one, that runs on Windows.

As to which you want to learn, it depends on where you want to target your career.

As a rule, Silicon Valley tech companies tend to use MySQL or PostgreSQL on Linux environments (and often a set of NoSQL data managers as well), with MS SQL and Oracle used less frequently. The non-tech world tends to use MS SQL or Oracle for back-office IT.