Saturday

Today has been a long, lazy day and it's dragging. (going to slowly slowly.) I'm going to complete my exercise. Now I want (to) a cup of coffee because I'm so (too) tired but I ....have no / am out of .... coffee. So, I'm sitting here looking glum..(with sulky face.)

You first sentence needed a subject so you should include 'today'. 'Dragging' describes something which is going very slow - we often say 'time drags'. In your second sentence, it could mean you are going to complete some physical exercise (keep fit) or write something. I am assuming it was to write this article. Your second and third sentences are a little out of synch, I think. Normally would we say 'now ............' and follow it with 'I'm going to .......' which means later. 

Saturday

A long lazy day going slowly. I'm going to complete my exercise. Now I want to have a cup of coffee because I'm too tired but I have no coffee. So, I'm sitting with a sulky face.