If you are going to use a program to solve it. What you need is not an analytical solution.
You should solve it numerically.

$\frac{d^2\theta}{dt^2}=-\frac{g}{R}\sin\theta$
can be break into two first order differential equation

$\frac{d\theta}{dt}=\omega$ and $\frac{d\omega}{dt}=-\frac{g}{R}\sin\theta$

You should know the initial condition $\theta(0)$ and $\omega(0)$

With Euler's method:
$\theta(dt)=\theta(0)+\omega *dt$
$\omega(dt)=-\frac{g}{R}\sin\theta(0)*dt$

It means that $\theta(t+dt)$ and $\omega(t+dt)$ can be calculated from $\theta(t)$ and $\omega(t)$

The time step need to be small enough to avoid accumuation of numerical error.
There are better numerical method to solve it. e.g. Runge-Kutta 4th order.
I think you are learning it right now? Right?