So an atomic bomb has several methods of killing you; physical shockwave, thermal radiation, radiation burst, and fallout.

Being inside a building will virtually eliminate risk from radiation burst killing you. Many people in Hiroshima suffered partial burns because the "flash" was partially blocked by an object, the exposed skin was instantly burned.

The blast effect creates what's known as overpressure. Again very deadly if outside and it will cause building destruction and damage. The blast weakens as it moves farther away doing less damage to buildings as it goes. You want to be inside to avoid overpressure.

The next is thermal radiation. Very quickly things immediately surrounding the detonation are going to spontaneously catch fire. Most people even inside buildings will be killed by the intense heat. However this drops off and starts smaller fires away from the blast. These fires can and do produce what's called a fire storm, which can rage for hours after the blast. These storms spread and suck in people and debris. Near or inside the fire storm zone is almost certain death. During the fire bombing of Desden fire storms were created and the fire burned so fiercely it sucked air out of the bomb shelters underneath it.

So if you're outside the firestorm area there are good chances you'll survive the initial blast. I'd say better than 50/50 odds. You absolutely need to worry about building damage and usually the bathroom with no windows is a safe place to be just like a tornado (similar principals at play). You may immediately have to combat fires and perform first aid.

As for "avoiding fallout": Fallout is a collection of radioactive particles that are both from the bomb and it's reaction and also from debris made radioactive. The primary concern is ingestion or inhalation.

If your house is intact: stay indoors. Seal up doors and windows with duct tape. Turn off the AC. The shockwave carries very little fallout especially compared to what's going to fall over the next few minutes and hours.

If your house is damaged and cannot be sealed. Consider seeking shelter at another house or if that's not possible, secure a single room. Duct tape, etc.

Now is the time to take iodine tablets if you have them.

For water, you have to assume the municipal water supply is contaminated. Your best options are bottled water and the water from the top of your toilet tank. The bowl water is a last resort but for cleaning or washing off not the worst.

Turn on a radio. Listen for instructions.

After the fallout has passed hosing down your house may not be the worst idea if rescue seems remote. Limit your exposure, use mask, wash off afterward and discard clothing. Try not to splash yourself with either contaminated water or wash off water. (Toilet bowl scrub down).

Eat only food that were sealed at the time of the blast. Once open discard.

https://science.howstuffworks.com/nuclear-bomb8.htm

http://nuclearsecrecy.com/nukemap/

https://en.m.wikipedia.org/wiki/Effects_of_nuclear_explosions