Homework questions should generally be answered by your teaching assistant...

In brief, though, the current comes from Ohm's law. U = I * R -> I = U / R.

The trick in this problem is that the voltage changes as the capacitor discharges through current, so you have to solve a differential equation.

The fact that you don't know how to find the current means that you haven't learned what you're supposed to be learning in class. Go back to the book and I'm sure you can figure it out!