How do handle concurrent DML operations

Hi all,

I am building web application by using Oracle data base.
Because different uders will manipulate the same data record at the
same time.
Could someone please help me for the following questions:

1. How can I handel the concurrently DML (insert, update, delete)
opertions on same data record by different users? Can I use
synchronied method in update, insert, and delete opertionns? Or other
better method ??

2. When user 1 select data from table , user 2 update or delete the
date in the same table concurrently.
Do I need to set transaction isolation level to
TRANSACTION_READ_COMMITED or refreshRow() to get the updated data?

thanks all.