If you do not have an oracle driver do steps 1 and 2 first.  Otherwise skip to step 3.

1) Download the jdbc driver from oracle.

2) Install the driver into your local maven .m2 repository. Example:

mvn install:install-file -DgroupId=com.oracle -DartifactId=ojdbc6 -Dversion=11.2.0.4 -Dpackaging=jar -Dfile=ojdbc6.jar -DgeneratePom=true

In the above example the ojdbc6.jar version 11.2.0.4 is used.  Replace the -DartifactId=ojdbc6 -Dversion=11.2.0.4 and -Dfile=ojdbc6.jar with the driver name/version you downloaded.

3) Add the dependency to your build.gradle file. Example:
dependencies {
    ...
    runtime("com.oracle:ojdbc6:11.2.0.4")
    ...
}

replace driver name and version with the one you downloaded

4) In spring boot add/modify the application.properties file to use your db url, username and password.  Example:

spring.datasource.driverClassName=oracle.jdbc.driver.OracleDriver
spring.datasource.url=jdbc:oracle:thin:@127.0.0.1:1521:orcl


spring.datasource.username=username
spring.datasource.password=secret