Tips for beginner player

Looking for tips on what to prioritise as a beginner player, im level 55 and so far have 13 trains slots and 3 international slots. I finished Mrs. Wilma and Johan contracts. What should be my priority when it comes to materials? Should i focus more on materials or just earning straight up gold. What are some locomotives i should invest into and are locomotive sets worth it even if they pull less wagons. Thanks