Probability is given by the number of 'things' you're looking for, divided by the total number of 'things'.
In your case, the things are marbles and what you want, are black marbles.
The first time to pick marbles, how many black marbles are there? How many total marbles?
If you understood what to do, you should get the probability of getting one black marble. Can you post what you get? When you're done with this, we'll see how to get the next black marble.