Not really a mathematica problem but here is something to get you going in the Wolfram Language.

```
eqn = x''(t) + (2 π 100)^2 x(t) == f(t);
ic = {x'(0) == 0, x(0) == 0};
f(t_) := Sin(110 2 π t);
sol = NDSolve(Join({eqn}, ic), {x}, {t, 0, 0.2});
Plot(Evaluate(x(t) /. First(sol)), {t, 0, 0.2})
```

Here I have made an oscillator with a natural frequency of 100 Hz and then applied a force with a frequency of 110 Hz.

The equation you are after is called eqn. The initial conditions are ic and the force has been defined as f(t).

You should be able to copy and paste that into `Mathematica`

and get the plot I have shown.

Good luck