As of late, atheism has been observed as less of something that can cause social stigma and more as a point of view that's increasingly being accepted around the world. Still, there are a lot of people that are having doubts, either being agnostics, or people who simply don't believe much in their religion anymore due to being open to other religions, not being decisive, and a variety of other reasons.
First things first, you cannot force anybody to abandon anything, due to, of course, basic freedom and morality. What you can do is recognize that people have the right to their opinion and you have a right to think that opinion is stupid or wrong. So the first step is that if the topic of religion or lack thereof ever arises, you shouldn't be afraid to express your take on the matter. Some people will get mad, some won't, some will pay attention and in the end that's the important thing.
As with anything there is, there will always be somebody who's worked on it before you have, so you can watch debates in which Richard Dawkins has participated (In the past, of course, lately he's been too immature), Christopher Hitchens, watch George Carlin's rants, etc., and learn the arguments, and know the usual counterarguments like Pascal's Wager, but always remember that the burden of proof lies with the believer. You can say anything exists if nobody can prove it doesn't.
Speaking of already existing material, you can post it on Facebook, share it on Tweeter, give it a thumbs up in Stumbleupon, that sort of thing which will help it spread. If the content's good, trust me, people will like it and a good enough argument can change somebody's mind, no matter the circumstances.
|Unless, of course, a man deliberately ignores you.|
Just remember not to bash somebody with logical fallacies, like ad hominem or anything like that. That sort of attack is for somebody who doesn't have anything to base himself upon. So, remember, it's not something you can force on anybody but worry not for the world's already moving to this state of mind - atheism has increased in the newer generations, not by much but it's formidable enough. So, go ahead, promote it a little. We can speed it up.