Set a planet's distance from the star and its launch speed, then release it. Gravity does the rest. Too slow and it spirals in; too fast and it escapes; find the balance and it settles into a stable orbit. Speed is shown relative to the exact circular-orbit speed for your chosen distance.
Circular speed (1.00×) gives a near-perfect circle. Around 0.7× drops into an ellipse that grazes the star; above ~1.41× reaches escape velocity.