3) How to set the leave application to be on the basis of half day and full day?

From OrangeHRM

A half day or a full day will be calculated based on the number of working hours per day.
The default work shift is 8 hours. Applying leave for a half day, means a 4 hour shift. Respectively full day means an 8 hour shift.

To change the work shift hours follow these steps:
1. Login as HRAdmin
2. Select Time -> Work shift
3. Press the add button to add a new work shift.
4. Give a name to the work shift and set the number of hours.
5. Save the the record
6. Now click on the work shift link that you created. You will be able to add/remove users of the particular work shift.