The best way to determine priorities is to look at the goals and objectives of your supervisor/manager and the company for which you work.
For example, if one of the stated goals for your company is "excellent customer service," you will need to look at your duties for the day in terms of how the customer would view the service delivered.
Another example is to look at the deadlines that have been set for various items on your "to do list." If there are three items with due dates and the due dates are tomorrow morning at 9AM, tomorrow at 4PM, and next Friday by noon, which one do you think needs your most immediate attention? Probably the one for tomorrow morning at 9 AM, right?
What if the item due at 4 PM needs to go to the printer and be back by 4 PM? You have to work backwards from the date and time it is due including all the steps needed to complete the project to determine when you need to start working on your piece of it. Let's look at an example.
Suppose you are in charge of planning the staff meeting for next Friday at noon. Lunch will be served. You have to do all the following:
A. Reserve the meeting room.
B. Order the food.
C. Send out email reminders to the staff.
D. Fill the ice bowl for the drinks.
E. Order the markers for the white board and the markers for the flip chart.
F. Send a request to the Audio-Visual Department for the Projector for the Laptop for the PowerPoint presentation.
G. Type the agenda that the boss gives you.
What should you do first and when should it be done? Can the entire list wait until next Thursday? Probably not.