Hi Sina,
Its difficult to answer this question unless I have the following details. There are quite a lot of journals available in Oracle Tuning.
If you could send me the folloiwng I can try to help you with some specific suggestions.
1. THe SQLs, Stored Procs etc. that run slow.
2. The Table DDL.
3. Soem info on the number of records each table will have.
Thanks and regards,
SS