Weaponry used in the war
Firstly the question is how 'dirty' the weaponry used in the war was, as anything other than clean bombs (https://en.wikipedia.org/wiki/Nuclear_weapon_design#Clean_bombs) or something like pure fusion wepaons (https://en.wikipedia.org/wiki/Pure_fusion_weapon) would result in many years of residual radiation and radioactive particulate. The radioactive particulate in particular can be a problem as it deposits and becomes embedded in the soil, effectively ruling out effective horticulture (Ie. The environment may be relatively safe to traverse but it would be a bad idea to dig up the soil for any reason)
Beginnings of Settlement
Assuming that the population in the subterranean dwellings also has a stockpile of equipment and resources (machine fuel, generators, etc) then the first step is the acquisition of resources essential to survival.
1. Water - An effect of nuclear fallout is a vast reduction in precipitation rates due to dust in the upper atmosphere (~45% reduction - http://academic.evergreen.edu/z/zita/articles/SciAm/GW_CC/NuclearWar2010.pdf) therefore it will be essential to bore deep ground water wells and check regularly for radioactive contamination.
2. Food - Since the soil will most likely be still contaminated, the safest way for production would most likely be something akin to greenhouses or buildings containing hydroponics basins and modified uv lamps. Depending on meteorological conditions and the nature of the nuclear weapons, there may be significant solar dimming or cold resulting from dust. Most of this however should have settled in 5+ years.
3. Shelter - The ideal would be to use the subterranean complex for the first generation of settlers, as this will provide ready made shelters as well as established infrastructure. Also there will likely be very little wood available for construction as a result of fire, low light conditions, and low temperatures etc. New trees will be not be suitable to cut as plants bring radioactive material from sub-soil to the surface, and cutting them could therefore expose humans to extra dosage of radiation. Quarried stone will most likely be the best and most readily available basic building material as it will be the least affect by radioactive fallout
Advancing from Basic Settlement
Once the basic elements of food and shelter are setup, establishing infrastructure to service the needs of the population will be important. For the first 500 years, there will be bountiful supply of metals to be reclaimed, smelted and reused for tools and infrastructure improvement.
1. Workshops and Fabrication - Initially a number of high volume workshops will be needed to fabricate everything from tools, to pipes, to struts, moving part mechanism etc. This will allow for work on the other projects to advance
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Water - Water will need to be distributed first from a central point (eg. well(s)), however as the water needs of the populace rise a distributed system of pipes will be needed. Pressure to push the water around the system will have to come from either high elevation reservoirs or water towers.
3. Waste - One of largest factors in the extension of the human life expectancy was the toilet. Getting excremental waste away from the places where humans will eat and sleep will prevent disease and parasite proliferation.
4. Roadways and logistics networks - As the settlement grows, it will become vital to be able to move people and goods around in a safe and efficent manner. Roadways and thoroughfares will have to be established, as well as methods for the movement of goods (Teamsters, delivery persons etc)
5. Power - Assuming there was no portable nuclear reactors etc stored for this eventuality (and the skills to safely use them), then the only viable means to generate power will be kinetic based renewable power generators. Solar panels will not be viable as it is a complex industrial process to create even minimally efficient photo-voltaic solar panels. The most suitable power sources will be hydroelectric and wind powered generators as these are relatively simple to construct and materials will be readily available for construction
6. Schools - While one of the lesser immediately important pieces, having a existing knowledge base to begin with is a huge advantage in getting started, but more importantly to prevent knowledge degradation, a school system coupled with an apprentice type system will be vital to ensure the preservation of knowledge especially around complicated technology and processes
7. Policing - Inevitably, with any larger population, some degree of criminality or behavior that infringes on the freedom of others will occur. Establishing early a system of fair assessment of crime and punishment will be important to preserve the necessary happiness and productive societal attitude (ie. Widespread lawlessness and infighting would destabilize the population to the point of self-destruction)
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