Okay. First, download steam if it’s not already done : http://store.steampowered.com/about/
And create an account too if you didn’t. But I am pretty sure you already did.
Once steam is installed, execute it and log on. It’ll show you the steam navigator. Select the “shop” button (shop text at the top left of your steam navigator) if it’s not already selected by itself. Now look at the top right corner of your screen : there’s a search bar. Type “Dirty Bomb” on it, and run the search by clicking the small magnifer button.
The results will show you Dirty Bomb as your first choice : click on it. It’ll bring you on the game’s steam page. You just have to scroll a bit and click on the download button.