Program, that chooses the best out of 10
I'd solve this exercise with dynamic programming. You should be able to get the optimal solution in O(m*n) operations (n beeing the number of cars, m beeing the total mass).
That will only work however if the masses are all integers.
In general you have a binary linear programming problem. Those are very hard in general (NP-complete).
However, both ways lead to algorithms which I wouldn't consider to be beginners material. You might be better of with trial and error (as you suggested) or simply try every possible combination.
I need to make a program in python that chooses cars from an array, that is filled with the mass of 10 cars. The idea is that it fills a barge, that can hold ~8 tonnes most effectively and minimum space is left unfilled. My idea is, that it makes variations of the masses and chooses one, that is closest to the max weight. But since I'm new to algorithms, I don't have a clue how to do it