As the other answer said, start by writing down what you know.
Now, assuming your formulas are correct:
also using ohm's law, which states: I = V/R (where I=current, V=voltage or emf, R=resistance)
Using ohms law you solve for V.
V = R*I or emf = R*I
So you should now plug in the formula for I, which you have as:
i(t)=I_0*e^(-t/RC) and multiply by R to get emf.
With this emf plug back in the first equation for charge.