You play Fallout for a while. You learn a few new things about navigating the hazards of the Wasteland. [+5 % to Outdoorsman skill. ]
Seriously though, most of surviving a large scale nuclear attack depends on if there is a warning; if you have the time and opportunity to get to a good or at least adequate shelter; how long you can stay in there with clean food, water, and air; if you have access to potassium iodide (to prevent thyroid damage if you receive a large radiation dose); if you have a Geiger counter or other method of accurately gauging when the outdoor radiation level return to tolerable levels (which could range from a week to several months depending on the location and severity of the attack).
The first phase is just surviving and it depends on luck, location, preparation, and what kind of attack it is. The second phase is about finding a long-term source of clean food and water and defending yourself (and your supplies). The main thing there is to figure out some way to organize you, your loved ones, and other people you trust into an effective fighting force.
This isn't to say you need to kill everybody in your way, but you can't let in every scavenger who might be part of a different gang of thugs, you can't let idiots in your group jeopardize your safety just to satisfy their sense of democracy, you can't feed all the people you come across because then you'll both run out of food. It involves arming yourself, keeping watches, finding a good destination (to ultimately grow food) or a good migratory path, and knowing when and how to integrate new people into your fledgling nation (until - if ever - relief arrives and you can or should reintegrate your group into a larger group/country).
You need to evaluate new comers, keep a watchful eye on them, make them swear allegiance to the group, and get them to commit to the group before you ever let them hold a rifle, etc. And you need to enforce military discipline within the group. This isn't to say that you need a strict chain of command adherence, but you would need to avoid stupid mistakes and to prevent decision making from devolving into the dangerous arguments that get people killed in zombie films.