[it's the first day/hour] [the apocalypse is kinda over already]
-> I hope it is clear to only band together with people you TRUST in case of the apocalypse.
-> Staying at the same place is a good idea. Set up night watches if possible or if needed (don't if there's more than 4 people otherwise you'll be very tired which is bad; still do if you're 3 people when there's a high chance of questionable individuums around.)
What you do now depends a lot on which kind of apocalypse it is!
-> If its a zombie apocalypse, you need to get away from densely inhabited areas quickly - but don't forget to quickly pack as much tools (the most important things! best tools are axes, rust-free (=stainless steel) knifes, saws) and food (ideally canned meat, noodles / grains and anything made of sugar!) to help bridge the first time.
-> If you alreads live far away from cities and such, there is a need to consider what your house is made from. If it's drywall and framing, the smartest thing would be to barricade every low-lying entrance and window you can find and, if there is any, move the REMAINING furniture in a way that it lays against the outer walls.
Set up your "main camp" in an upper floor and move food and tools there. Barricade the stairs, too. Take a ladder if you have one and place it at the window of your room as an entrance - REMEMBER TO PULL IT INSIDE AT NIGHT!! Remember that crawl spaces below your house are not ideal - if you have trusted company, use the safest house!
->If your house is made of concrete, plaster and stone, barricade the lower windows and doors as well as the stairs. There is little need to reinforce the walls, use the furniture to barricade the more vital areas. Otherwise, continue like in the paragraph above.
-> If it's an atomic apocalypse, you life in / very near a big city, and aren't nuked yet, pack tools and food and GO! NOW! RUN!
-> if you live far off (> 30km / 20mi) from the next important city or vital military/industrial location that might get nuked, bunker in your house's cellar or in a middle room of the ground level. Seal doors and windows with tape if you can. Move food and tools and most importantly your bed far away from the roof (and the outer walls). This should help keeping away a lot of the more agressive radiation.
-> It is good practice to still bunker up your house to discourage potential intruders.
Proceed just like in upper instructions. A zombie apocalypse doesn't end very quickly usually.
-> If a bomb exploded near you, remove all clothes expect vital ones (if it's warm enough for it and is likely to stay so for a while). Breathe through some cloth to help keep dust out of your system. Try getting into a car and driving straight away from ground zero - ideally into a less inhabited area. Pack tools but ONLY CANNED foodstuffs as others are likely contaminated. If you cannot get into a car and the explosion has happened less than two days ago, move inside a house and proceed like in upper instructions. -> If ground zero is more than 30km / 20mi away from you, move camp inside as told in upper instructions. Go out for supply trips in the DAY, if you are at least 3 people always go on trip in groups of 2 (or if you are than 5, 3). Never leave less than 1/2 of your total group at camp!
Now that the first steps are covered, you may continue to: how to not starve