Logging is your first line of defense in terms administering, debugging and monitoring any part of the data center and especially the application server. WebLogic generates a number of very helpful log files for that reason. In addition WebLogic also provides ways to robustly manage these log files in terms of tuning things like log rotation and filtering. Ahmed Aboulnaga introduces some of these capabilities in a recent article on OTech Magazine (his article is mostly focused on the admin console).
The most detailed and up-to-date way to learn about WebLogic logging is always of course the WebLogic documentation. For example a couple of important logging aspects the article does not get into include configuring the logs themselves as well as easily viewing the logs through the WebLogic console.