There are endless ways to lift a ball high enough to get your marble machine running.If you can get it to the top, gravity will do the rest. Take a look at this awesome video to get a few great ideas for lifting the balls of your own marble machine.
source/image: Wood marble machine
This is a marble machine with a lift that lifts three or more marbles simultaneously, and releases them to three separate tracks. Two of those tracks are connected in series, so a marble will therefore alternate between those tracks.
The third track loops back onto itself, so to have traffic in all three tracks, you need at least two marbles.The machine has been designed for 8 mm balls as marbles, and the machine prints completely without supports if you have a well tuned printer. 8 mm steel balls will work fine.
If you sit down and take some time, you can also build machines like the ones below. All you have to do is practice. It takes a little trial and error to get some of these ideas working right, but that is half the fun of making these machines.