Dumb newbie question

Bugged me too... Try assigning a project... I think(!) it only uses up your social production and not military, so your social production is going to waste... I usually assign a research project or birthing subsidies.

Yes, scroll to the bottom of the improvement list and choose a project: Economic Stimulus, Birthing Subsidies, Cultural Festival, or Research Project.  Note that some have to be unlocked through research.  If you want a planet to focus more on military (shipyard) production, use the planet's govern window and move the manufacturing slider (not the wheel) to the left.

When something is added to the queue, such as an upgrade to an existing building or terraforming, the project will move to the bottom of the queue.

Oh, and if you like, you can command an idle fleet to guard or sentry and you won't have to pass every turn.

On setting a project, your social production will go towards the project.   So, for example, if you have a heavy manufacturing planet, if you need research rather than buildings or ships at the moment, you can set your slider all the way to the right (100 per cent social) and set a research project, and set your wheel to point to 100 percent manufacturing for the colony, and all that manufacturing will go to research.  In the case I am talking about, a planet with lots of factories, you will actually get more research this way than by pointing your colony wheel to 100 percent research.   Of course more research would be made by building lots of labs and setting 100 percent research, but I am talking about how you can manage when you have a short term goal to finish a certain tech, for example, before you start building ships.   Get the tech, set the slider back to 100 percent military, and crank out ships, if that is what you need.

Same for an economy project, birth rate project, or culture as Turkwise suggests.   So there are lots of choices for long and short term purposes.   Interestingly this means that manufacturing capacity is the most flexible mode of production, as compared to research, economy, or culture buildings, because the latter types of buildings can't be directed toward a project.  So imo building a couple of factories is always a good start on a new colony.

Now  if you have a project selected, and you finish a tech that upgrades one of your buildings, and the colony is set to auto upgrade, it will build the upgrade then return to the project.

It may seem a bit much, but actually it doesn't take long to get the hang of it and it works very nicely and is much easier to handle, and more intuitive than it was in GC2 --- being able to set colonies independently, for example, greatly increases the range of possibilities.   

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At this point I would recommend using guard rather than sentry to hold a unit in place.   On sentry, it appears to me that even the approach of a cargo ship will activate the unit.   On guard it doesn't.   

That was the behavior in GC2, so it seems to have carried over into GC3.  I have never found a use for sentry in my playing of GC2.  Guard is a great thing.

I suppose if one used apicket line of ships to detect hostile fleets sentry might have a use, but because of the different color, and the possibility of building long range scanning ships it is probably moot.