Well, he starts of his poem by describing scenes that are very low-key, passive, and lifeless. For example, he describes the evening to "a patient etherised upon a table," a description that brings to mind someone passed out because of anesthesia, right before a painful operation. That could symbolize his own mood; depressed, lethargic, dreading the painful encounter to come. The dreary mood continues as he describes "half-deserted streets," streets that are not bustling with life and activity, but are deserted and alone, just like he is probably feeling. He probably feels like he is never part of the bustling life of the party, but on the deserted outer fringes. He compares the streets to "a tedious argument," which gives the feeling of being upset but not in a passionate way; it is tedious, burdensome and tiresome to argue. He then describes the "yellow fog"; fog is dark, dreary, depressing and confusing.
He spends quite a bit of time describing the not-pretty parts of town, the fog and empty streets, the feeling of being numbed to all around him. These city scenes and descriptions symbolize his mood of depression, dread and lethargy. Modernist writers often used the environment, or the scene, as a symbol for how the characters were feeling, and Eliot does that here too. I hope that helps; good luck!