First, welcome to Worldbuilding, <PERSON>!
Let me preface this by saying that this answer <PERSON> gives about volcanism is a good start, and it was my first thought, as well. But in considering the additional details you've given, you may want to go another route (that is, depending on how realistic you want to be). I'll expand on some of the finer points below.
Now, do you want the surface of your planet to get really hot, or really cold?
Cold
If you would prefer the surface environment of your planet to end up being quite cold, then (paradoxically) a good place to start is with increased volcanic activity. As mentioned by @rek, a volcanic event on par with the Siberian Traps would indeed be catastrophic. However, the focus then becomes: How fast do you want your extinction-level event to occur, versus how long before people can safely live on the surface?
If you want your people to see this catastrophe coming far in advance, giving them plenty of time to build bunkers and prepare, then one way to do this would be to start with a series of small eruptions (or a couple of large ones) that in turn give volcanologists indications that such activity is poised to increase in severity and frequency. Respiratory issues will occur in places, but by-and-large, they won't be a major problem by themselves in the short term.
Alternatively, if you want volcanic activity severe enough to cause a massive, unexpected extinction-level event quickly, then (without protective gear), the surface of your planet will be uninhabitable for a very, very long time. The P-T extinction event took millions of years to unfold. In order for your wasteland conditions to appear in short order, you would need something much more severe. For starters, there will be a lot of fire. Molten rock and burning ash will rain down over your planet like fireballs and flaming confetti. All the resulting ash and CO2 would certainly cause the atmosphere to become too toxic for oxygen-breathing lifeforms, nor would you be able to reliably grow anything using traditional methods due to the reduced levels of sunlight.
Gradual volcanic event:
Even some of the largest eruptions in history, while disruptive, didn't result in any significant long-term repercussions by themselves. 591
The ash these eruptions threw into the atmosphere caused global cooling of a few degrees; winters were rougher for a couple of years, with record snowfalls and strong blizzards occurring. Naturally, a string of such eruptions in close succession would increase these effects.
So, if a) your eruptions are relatively steady and strong, and b) there aren't many of them in total and/or spread out over too long a time (say, a few months or less), then once the volcanic activity stops, the planet will return to normal over several years, or perhaps decades (again, depending on the exact details regarding the climate and topology of your planet). However, if your increased volcanic activity goes on for too long (say, months to years), you could plunge your world into an ice age. Once ice sheet growth and ocean temperature decreases become significant enough, then even without a single eruption, it'll be a cloud-covered, snow-smothered wasteland for centuries to millennia, at best.
Sudden catastrophic event:
If instead you want a single, massive incident of volcanic activity to wreak havoc on your world with repercussions felt for an extended period of time, then the ash itself becomes a more immediate concern. This would be the route to take if, say, you didn't want the inhabitants of your planet to see the event coming, or you want them to have the bare minimum amount of time to prepare. Portions of the surface near the eruption zone will be in flames from burning debris, and large portions of the human the population (if not all of it) will find difficulty breathing. If you also want marine life to die off, then there will need to be enough CO2 in the atmosphere to choke off nearly everything living on land (humans would find the atmosphere toxic in this case, as well). Then, cue the blizzards.
Hot
A severe increase in solar activity could compromise or overwhelm your planet's magnetosphere. The increased ultraviolet exposure would turn your world into a desert. People would be able to walk around on the surface and breathe just fine without assistance, but they'd want to stay out of direct sunlight as much as possible. In any case, the increased exposure to UV and cosmic radiation will undoubtedly result in a spike in cancer incidence.
Increased solar activity can occur naturally. It's up to you whether your scientists get any advanced warning or not; stars can be fickle. 208