Its not that hard, just think of the 5 senses (sight, sound, smell, touch and taste) and describe what some or all of those things would pick up in any given moment of your story.

Assassin sat at his computer, eyes darting around the screen infront of him franticly. He smelt of sweat and body odour like he hadnt showered in days, which wasnt entirely true, he had gone for a swim the day before, but only briefly. His fingers darted over the keys, their silken smooth surfaces caressed by his almost girlishly soft hands as he played his game, the sound of gunshots and explosions going off constantly. With a smack of his lips, he reached over and grabbed the bottle of coke, swigging on it and putting it back quickly in one of the brief lulls in play. He'd been at it 18 straight hours and had almost finished the first stage. At this rate, he'd be finishing the game by next month. Assassin had never been much of a gamer...

See, just describe the senses and youll be A-OK as long as you know where your plot is starting, and where you want it to end. I wanted to make myself sound like a slob playing a game... which is sorta true! :P