On large maps it's often difficult to start, especially if you look for production of end products. But you have to see it as a role playing game. You start small, and you will look for the few profitable routes. Often they are the shipment of primary resources (always pays), and the transport of passengers between nearby towns.

Little by little you will have the finances to invest in more interesting product chain segments, and place your own industry plants at strategic locations. Once the wheel of fortune turns, the ball is rolling.

So. start small, pick the few opportunities to make a little money, invest and grow slowly. Raw resources are your early friends. And, if it was easy, it was no fun. ;)

Thanks for your comment. As providing end products to the the cities are quite impossible in early game, the cities might growth slower as usual, isn't it? And to make more space for putting new industries, would it better to start the map with less density?

I suggest to create a game using the map editor. It's more effort, but the optical end result - how the towns are placed, where the industries are - looks more realistic then.
Even the road layouts are much better, with more authentic serpentines etc.