First off, new name. Sounds like
Fallout: New Vegas. Looking over the plot of said game, there's a lot you should probably revise because of similarities between your proposal and the actual game.
ProjectATL wrote:atomic blast like a nuclear bomb
Just to point out, I don't know what else could make an atomic blast
Moving on: don't ask for help (beyond advice) here. why? you're not going to find anyone willing or able to help.
To start, write up a full story, check to make sure you're not subconsciously stealing story ideas, and then move on to the design phase. Establish a good setting, find out what each map or area will need, and then go from there. I don't know whether you're using vanilla (unmodified) Sandbox or if you're using custom content, but make sure that you get a good idea of what mode you want to use and how to work with Sandbox. Once you're ready, you can start making maps.
! It's about a man named Terrence "T" Miller. His life is great since he has 3 kids, a wife, and is enjoying a great job as a fireman. It's also 2 days away from new years day. T and his family travel to new york just to enjoy the huge countdown into 2020, but right on 12:01, an atomic blast like a nuclear bomb occurs turning Earth into a lifeless wasteland. T is knocked out for 3 years and finally awakes in a comfy bed at some broken down treehouse. He was put there by a woman named Alice Garcia. Alice informs T that his family is dead and that there is a safe place for all survivors in Las Vegas, now called New Vegas. T is sad but picks himself up and begins a dangerous journey to New Vegas kicking mutant, rebel, and bandit butt. This is a shooter game and I call it Y2K20 Act I. Thnx for reading and tell me what you think! 
