Very good question. There are many methods, but for me its the first thing I do early in the morning. I use project management tool to help (in my case I use trello.com). With trello it arranges your tasks in card like fashion, thus making it easier to monitor and organise.
I break into several columns of cards from left to right, with the left being the most important and right being the least. So obviously the left most column of cards would be named "Clear by Today!" and the next right to it would be "Urgent", then "Important", "not critical", "future", "KIV" and finally "Completed"
Each of these column would have multiple cards in it (one task per card). My target is to finish off all those listed in the "Clear by today" and if I have additional time, I may go to the next column
Within each column of cards, you could also click and drag each card and sort them out based on priority (from top to bottom, with the top most card being the most important). This makes it easier for you since you just have to go through from top to bottom for the left most column set of cards
With regards to identifying which is important and which is not - it would depend on 2 levers - impact and urgency. Those with high impact and high urgency has my top most attention, while those with low impact and least urgency naturally would get the least