SQL tuning is not a simple quick fix
You need to look at the queries being run and determine if they could be better written. The default MySQL install should be OK for most things. Quality of the query and lack of correct indexes is usually the biggest problem with any SQL system.
The slow queries log is probably a good place to start.
Have a long read of the mysql.com web site, there's plenty of reading material.
If you want to tune, then you'll need to understand what the database server is being expected to do, then tune to your requirements.