Download the Oracle Instant client from this link in case you want SQL Plus.
Now make sure that you have SQL Plus exe on your path.
If you want an alternative, use SQLDeveloper from this link or SQLcl
Alternatively, you can also use Oracle SQLDeveloper from here or SQLcl. You can prefer SQLcl because you can connect to any Oracle database by using EZconnect syntax
sql.exe usernamepassword@yourdatabasemachinename:yoursqlportnumber/databasename
e.g.,
sql barry/omagh@devbox:1521/DEVDB1
If you want to get more insights into SQL, check out this SQL Course from Intellipaat.