The gist of it is: nothing comes really easy in the post-apocalyptic Slav world. You need to work for your money - and this game takes an oldschool approach: no tutorials, no hand-holding; here's your truck and some very broad directions, learn as you go.
Think old(-er) "rags to riches" games like original "Elite"-s, "Privateer" and such. None of that "follow the map marker and press X to git $$$". ;)

So, what do you do?
Well, start trading, of course!
Buy stuff where it's cheap, move it to where it's in demand, and sell high!
We'll get to that in just a second, but first let me give you some...

• First and foremost, after you're done with the introductory delivery from Jacek, DO NOT be tempted to fill your gas tank up to the cap. Fuel consumption is an issue to monitor, but contrary to most of the games out there your clunker does NOT burn 20 liters of gasoline to move three paces. Actually, a full tank should be enough to circle the whole map, and since you'll probably stick to the roads most of the time (and thus burn less fuel than while going offroad), so there is really no pressure for topping up the gas tank every time you get the chance.
  So fill up the jerrycan you've been given by your neighbor and keep it close as an insurance measure. Then only fill up the tank to about 50 liters, or 1/3rd. That's plenty enough to get you started, and you'll have more capital for trading.
  
• In a really anti-Slav manner, equip yourself with a 6-pack of water to stay hydrated, and a single can of pork in case of an emergency. Then, eat only when you're making a delivery to a town. An apple a day keeps the doctor away, a couple of them keep you fed very cheaply. And most routes aren't all that long to get dangerously hungry.
  
• Keep your (minimal) provisions in your truck cab. Do not waste bed space for stuff you probably will not need, like the toolbox. Leave it all in the garage and visit only when you need to repair or swap some parts.
  
• Abuse your backpack. That's 10 more items you can move and definitely won't come flying out if you have an accident (but you can't stash pallets and other heavy goods in there, so there's that; packages and even barrels are fair game though).
  
• Be a careful driver. Don't go charging blind if you don't know the road - one sneaky bend in the road over a crest will send you flying into the dirt and your cargo all over. That's money down the drain for repairs, but most of all picking and packing wastes a lot of precious TIME. You're not making any money while gathering your stuff after a rollover. As of now, there are no means of securing the cargo to the bed, such as straps (but the dev said he's looking into it).
  
• On a related note, you can go fast if you have confidence in your driving skill, but don't over-rev the engine. Higher RPMs mean more fuel burnt, and if you overheat the engine you'll start losing coolant and oil (a lot faster than the usual wear and tear). So shift early and don't keep the pedal to the floor for too long on the top gear. Unless your truck is struggling uphill or offroad, try to stay below 6,5-7K RPM at all times.
  
• ... but watch out when driving down some steep and/or lengthy inclines. It's easy to capsize on even a minor bump in the road (and depending on your load the truck might become unstable very easily), but also to overrev the engine (yeah, engine braking sucks at the moment). So pump the brakes gently once in a while (so as not to lock the wheels, and to keep the cargo from shifting too much) and keep your speed below 70 km/h or whatever you feel comfortable with. Also remember that you can manually apply the clutch by holding X or switch to neutral gear by pressing 0 - that way you can coast downhill as fast as you wish without exploding the motor.
  
• Avoid the mountain pass route like the plague. Yeah, it's there, yeah, there are some nice views up there - drive it once in every direction to satisfy your inner completionist and don't ever return - because it just takes so LONG and exposes you to unnecessary dangers. Take the southern route and grab some deliveries on your way - you'll arrive quicker and be more wealthy in the process.
  
• You can swap in a diesel engine, which has a bit less power but burns less fuel, which is also cheaper than gasoline - so more economical in general. But the rusty one you can find for free (see other guides or check out YouTube videos for details) is pretty much crap compared to what you have when you start, and when you finally have enough money to buy one in better condition, it's pretty much pointless to penny-pinch. So, a balance issue, I think.
  
• If you dabble in illegal goods, be sure to stay well away from the cops -- and keep your cargo ONLY in your backpack.
  
• Check your truck's fluid levels every so often and refill as needed. To check the coolant level you have to pop the hood and aim at the radiator cap. Fuel and oil have indicators on the dash, and aiming at their respective caps works also.

For starters, I'd recommend hauling coal from the coal mine to Eastport. When heading back towards Kolovy, be sure to swing by Jacek's and check if he has a delivery heading that way. (As of time of writing, that is ver. 28/01/22, contracts are always the same and respawn on game reload... You can abuse that while it lasts.)
It should net you around 600 SD per bag (but don't pack too much, the bags like to fall off the shallow bed, if you don't drive extra carefully), plus the 14K from Jacek's delivery to cover the expenses.
Once you have around 150K to your name, you can head to Oasis Forestry, grab two pallets of logs and deliver them to Balarsk Sawmill for 25K profit, immediately buy two pallets of planks there and deliver them to Lupova building materials for 27K profit. From there you can either repeat the process by heading back to Oasis Forestry, or go to the Oil Field and grab 10 barrels of crude oil to deliver to the refinery for 75K profit, which is also nice because you can load and unload them without utilizing the forklift.
There should be a contract at the refinery to deliver 15 barrels of diesel to Sterlikin gas station, which is pretty much just around the bend - easy money. Also grab as many barrels of diesel you can afford, and after delivering to Sterlikin carry on to Komsodrinsk for 9K profit per barrel, remembering to grab a delivery or two on the way.

From there, the world is your oyster! Generally the loop is:

1. Go to your pickup point
   
2. Grab your cargo
   
3. Check if there are any delivery contracts headed in the same direction, if you have the space
   
4. Drive to your destination, swinging by nearby settlements if they're easily accessible (so probably skip Magyarfalu since the climb takes too long) to resupply and check for contracts
   
5. Unload your cargo at the destination. Using your hard-earned money, drink, eat, smoke, sleep, repair, refuel and whatever else you need to do in preparation for the next trip
   
6. repeat

But wait, that's not all!

"But Grzeszyk", I hear you say, "I don't have a slightest idea where to buy what and where to take it for profit".
Well, I did the legwork for you so you don't have to.
Head on over to this link[docs.google.com] for a *mostly* complete spreadsheet of what you can buy and sell where and for how much money (contains some very minor spoilers). Analyze, calculate and plan to your heart's content! :D

I promise to update this guide and the aforementioned spreadsheet as long as I have interest in this game. No idea how long this may be :)
Anyways, both are provided as-is, without any warranty on accuracy and correctness of included information (although I try to do my best).

The spreadsheet icon used for this guide's title ("branding") image is CC-BY-3.0 The Noun Project / Iconsparty[thenounproject.com]