I do not understand your question.

The interaction force between any two particles are model with relation: $\vec{F}=-\frac{GMm}{r}\hat{r}$ where $r$ is the distance between any two particles. (because it is 2D model so force is proportional to 1/r).
The gravitationf force should be model with $\vec{F}=-\frac{GMm}{r^2}\hat{r}$ for 3D simulation.
And particles are generated at random position with random velocity.