Pack light: Bring a few clothes and handwash them. With a small backpack, you can easily spend a whole day hiking or sightseeing and then take an overnight bus to another location. With a big backpack, you will walk slowly, sweating in the sun, and be an obvious target for every tout and every thief in the area.
Instead of bringing too many warm clothes, put on several layers of tshirts and pants if it gets cold.
Unless you are going somewhere very remote, you can always buy things at your destination. For a long trip, it is better to bring too little than too much stuff.
In wet weather, keep your things in a garbage bag inside your backpack.
GEAR
People have lived in places like the Andes, the Amazon, or Sahara for thousands of years without Goretex jackets or hiking boots. The only people you see wearing "outdoor gear" in these places are tourists. Unless you're doing some extreme expedition, you don't need any expensive items. High-tech gear is never as good as advertised and always has its drawbacks.
That said, if you want to spend money, spend it on durable, lightweight items.
The best shoes for hiking are usually sneakers or sandals. Unless you're on snow or in a jungle, hiking boots have mostly downsides: they are heavy and stuffy, the thick soles result in worse balance and a worse feel of the tarrain, the waterproof material will eventually let in water and then take very long to dry. I don't understand the purpose of "ankle support" for people with healthy legs - it restrains movement and makes you more likely to slip or trip over on uneven surface.
If you choose to buy "outdoor clothing", find something normal-looking, without a visible logo or reflective parts. Then you will feel less like a tourist when wearing it in a city.
Goretex and many other water- or stain-repelling materials contain PFAS which are carcinogenic to humans and harmful to the environment.
Get a 1$ rain poncho or a poncho-tarp. Alteratively, rain jacket + rain skirt. Shorts + sandals are a good outfit for rain - they dry quickly and if you keep walking you won't get cold.
Get a space blanket - you probably won't use it but it costs 1$ and weighs almost nothing.
People laugh at me for using a compass everywhere but it's reliable, fast and doesn't need a battery. Get a tiny one for 1$.
Hat + sunglasses + long sleeve for the sun.
Plug adapters: there are many types of sockets but you can survive anywhere with only the "EU" and "US" plugs (and the "AU" plug in a few countries). The "UK" socket can accommodate a EU plug with a bit of effort. Get a simple, small adapter (like this one) as these are the most versatile.
CAMPING GEAR
Don't bring camping gear "just in case" if you plan to mostly stay in hostels. It may be cheaper to rent it at your destination than to travel with bigger luggage.
Instead of bringing a tent, consider a bivy bag, a hammock, and/or a tarp, especially if traveling alone. They are lighter, less visible, and can be set up in places where a tent can't be, e.g. in a dense forest or at a bus stop.
A mattress may be necessary in cold weather (you lose heat to the ground). If you don't have one, camp on thick grass, lie down on your clothes and your empty backpack.
For some gear recommendations, see here or here (or here if you're Bill Gates).
HIKING / CAMPING FOOD
I usually only eat cold food when hiking alone as it doesn't bother me, takes less time, and saves space in my bag. If you'd rather cook, a good lightweight option is to make an alcohol stove from a can (simple design here). Use high % alcohol, e.g. denatured ethanol, and don't forget a lid for the pot/cup (or use aluminum foil as lid).
Prioritize dry, high-energy food. Avoid food with a lot of packaging so you don't have to carry a lot of rubbish.
Oats are great and don't need to be cooked. I mix them with cold water in my water bottle and drink them after around 15 minutes once they soften. For taste and nutritional value, you can add powdered milk, sugar, instant coffee, coconut flakes, milled flax seeds, rasins (prepare the mix at home).
Other good options: cheese, hard-boiled eggs (should last several days if it's not very hot), tomato paste in a tube.
If cooking, couscous or ramen noodles as they take very little time to cook. Add a bouillon cube and/or dried vegetables, e.g. beets, fried onions, seaweed, mushrooms.
For multi-day hikes, make sure you have sufficient salt and fat. Salted peanuts or potato chips are a good option.
Eat plenty of fresh food at the start of your trip and whenever you find a grocery shop.
Do a bit of research to decide how much water will you need to carry. Are there many creeks on your trail? Is there a drought? Will you need to purify the water?