Start learning to grow crops now. If you have a balcony with any amount of sun, there's at least some kind of crop you can grow. Potatoes in a grow bag are your best option but they require full sun. You can attempt to grow
sunchokes in a similar way in partial shade, or you can set up a normal pot and try beets, broccoli, garlic, or lettuce. You likely won't be growing enough food to sustain yourself, but you'll be learning valuable skills for gardening (
soil pH,
avoiding powdery mildew,
diagnosing issues based on leaf color,
seed saving) that will transfer once you bug out to somewhere other than the city. If you end up cultivating your own line of heritage crops that do well in your area, you can bring the seeds with you once you bugout.
Basket weaving. This can be done with (pictured above) willow, pinestraw, reeds, and vines of all sorts. Depending on your location, even in suburban to urban areas you can forage for the necessary materials required to get started making baskets. The link above also has places where you can buy materials if you can't forage them. Foraging the sticks in the first place is the only part of this which must be done outside. Think about how often in your life you use baskets, buckets, cups, etc. Plastic disintegrates in the sun, and people will always need baskets. After SHTF, this is going to be an extremely important skill.
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Image source)
Learn to make your own basic clothing. At the very least, learn to mend the clothing you have. For an urban prepper, you're likely not going to have access to new textiles. So you'll want to focus on creating clothing by combining, repairing, taking in, and letting out existing clothes. You can technically start this with nothing more than a needle & thread, a threadripper, and some clothing you bought from the thrift store.
Brewing & fermenting. Whether it be beer (pictured above), mead, kvass, wine, kefir, yogurt, sauerkraut, pickles, or sourdough, fermenting is an extremely important skill for food preservation. Most of the tools to get started fermenting your own foods are easy to do in an apartment kitchen.
While all of these are great skills to learn, none of them matter
if you can't find food to eat in the first place. Urban cities are WAY above the natural carrying capacity of the land they're located on. You simply cannot grow enough food in an urban environment to survive long-term. Should supply lines be disrupted and food stops getting shipped into the cities,
you must leave. You must go to where the food is. These are skills to learn while you are actively living in a bughive, but they're all pointless if you don't have plans for how to leave the city and where to go when SHTF. There's a couple auction sites online that
sell incredibly shitty land for cheap, but it's better than nothing. Ideally, you should get a plot of land outside the city you're currently in, and start working on turning it into your bugout shelter.