clojure.tools.logging requires that you configure logging the java way, which will effectively log4j as the underlying logging framework, most of the time (if you add it/have it as a dependency) and so eventually the only you need to do is to have a log4j.properties file on your class path with the following content:
log4j.rootLogger=DEBUG, console
log4j.appender.console=org.apache.log4j.ConsoleAppender
log4j.appender.console.layout=org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=%-5p %c: %m%n
While you can keep going the clojure.tools.logging route, these days quite a few people enjoy timbre.
With timbre, you add it to your project.clj with:
[com.taoensso/timbre "4.1.0"]
Then require it with:
(require '[taoensso.timbre :as timbre])
Finally use it with:
(timbre/debug "hello")
; will print
(def example-config {:level :warn })
(timbre/merge-config! example-config)
; update the configuration
(timbre/debug "hello")
; will not print.
For more detailed configuration options, see here.