When placing planets in a simulation, make sure you have the "orbit" button enabled. This allows you to place objects in orbit of a star, moon, asteroid, etc.
First, place your planets in the order you would like them. Next, select a planet and then right click on it, this will open the properties panel for the object. The default tab it opens to should be "Basic." Scroll down until you see the header "Orbital Elements."
The parameter you're looking for is "semi-major axis." Semi-major axis is the distance an object is from its parent body. This parameter will allow you to evenly space each object in orbit. For example, a planet spaced out every 500,000km.
I would recommend using the grid when placing your planets. It should be enabled by default when in orbit mode. However, press "G" if it isn't already enabled. If you want to make a vertical line of planets, the grid is very useful. Don't worry about trying to get the distance even between each planet, do that after, using the technique I mentioned above.
Let me know if this answers your question. :)