What actions-factors you need to take care while creating index?

Creation of index on table depends on size of table, volume of data.
If table size is large and we need only few data while selecting data then create index. How frequently column is used in where condition, its also important.

if table has already a primary key, then create a unique key for the required column. Creating a primary key or unique key automatically create unique index on the column.

It is also not advised to create so many indexes as it impacts on query performance of DML statments as every DML statment need to go through with various index and table segments simultaneously.

We can create index or rebulid online also. It enables us to update base tables on the sametime.

creating any index parallelly is not supported while creating and rebuilding index online.

CREATE INDEX idx_emp_mgr ON employee (empid,mgrid) ONLINE;

/*+ APPEND */ hint in serial & parallel mode in oracle.

/*+ APPEND */ hint enables the optimizer to use direct-path insert particularly in insert statements in oracle

In serial mode, direct path can be used only if you include the APPEND hint.

Conventional insert is the default in serial mode. 
It means that NO UNDO will be written. 

UNDO is backed by REDO, so the REDO volume will be accordingly reduced.

Direct path insert is the default in parallel mode. 
In parallel mode, conventional insert can be used only if you specify the /*+ NOAPPEND */ hint.
data is appended to the end of the table,rather than using existing space currently allocated to the table, means after HWM of the current block. 

Direct-path inserts are considerably faster than conventional inserts.