Oracle 11g database- performance enhancement and sql tuning methods My production database(oracle 11g- windows platform) is growing(it's about 200 GB now) so that the risk of performance issue is also increasing.

Please give me some tuning tips used to follow by a DBA for high performance. What are the methods to follow while tuning the sql queries.

Also how to Identify the queries that are consuming considerable resources.It will be helpful if anyone can share the useful link of the sites mentioning about the same.
I'd recommend reading Oracle's official performance tuning guide for 11g:

https://docs.oracle.com/cd/E11882_01/server.112/e41573/toc.htm