I don't know about the LDS, but, unless you are more sure than everyone else about the afterlife, I guess each one of us will turn to help from whoever we believed in, in our earthly life. So whereas most will know which door to go and knock on (what they find therein is another matter) in your case it appears you won't have a door to knock on. It's not too late yet. There's no shortage of options so why not pick someone at random or make a rational considered choice, that's why we have the grey matter. You might have to sacrifice something in this short life here as a result of your choice but it might pay off in the Great Beyond, you never know.

First of all, since as an atheist you have not accepted Jesus as your Savior and have not accepted baptism by one who has the authority to perform that ordinance, you will have to pay for your own sins. Where you go once you have paid for your sins will depend on the kind of life you have lived. If you tried to live a good life, then you will go to the Terrestrial Kingdom. If, on the other hand, you have not even tried to lead a good life, then you will go to Telestial Kingdom. We really don't have much detail about what these kingdoms are like other than that they are still better than that which we have here in mortality and that the occupants don't get to enjoy the same familial relationships that are possible in the Celestial Kingdom. Those that fail to receive the ultimate reward will also have to spend eternity living with the knowledge that they could have has so much more if they had just made better decisions in this life. This is a general statement of what one can expect in the after life. However, where any individual will go is ultimately between them and God. He is the one that will ultimately judge all of us.