Your question is quite vague, but the problem is quite common. Tuning databases, especially Oracle, is quite a specialized field. Number one, make sure that all of your tables are 'analyzed'. If they are not analyzed on a regular basis, the optimizer cannot make the best decision as to the most efficient way to retrieve the data. One of the most effective ways to tune the database is to look at the queries that are causing the problem. Add indexes to the fields that you are keying on in the problem query. Make sure that you 'analyze' the table after creating the index(es). If this still isn't enough, do an 'EXPLAIN PLAN' on the problem query, and see what path the Oracle optimizer is using. If it is not using the index(es), you can add hints to the SQL in order to force a certain path....
It boils down to a lot of trial and error...
Good luck!!