In the players menu is a "Freeplay" checkbox that allows you to play without worrying about money. It can be used for test games to experiment with things. You'll still see the money - just doesn't matter if you go negative. Great learning tool.
Frankly, it's how I play - I use Simutrans as a sandbox. Set goals like "Transport all the passengers" or whatever. Or just build my highways and connect cities.

But also means I don't have many good tips, except:
1. Industry is easier for profits on most paks - because you set up a connection and you get money.
2. Passengers do best when you serve all of them for all of the map - serving a single city is usually not profitable. So you have to build up money and then serve enough passengers to make some profit.
3. Trains are harder to set up and not have conflicts that are difficult to resolve (sending one or more trains to depot, fixing track). Trucks are easier, but generally have less throughput.