How to modify MySQL 5.5 settings that were set during the installation? Especially the one that defines how MySQL is gonna use server's CPU. I'd like to boost performance. Also, what are other ways to boost MySQL performance? (Maybe some code optimization hints?) Anyway. Any information will be useful. Thanks in advance.
Use InnoDB tables and configure it properly.  This site is a great resource: 

http://www.mysqlperformanceblog.com/