Alaric101

You should be able to hover over the cruiser button and it will say at the top of the info card what the reason is (probably not researched). Also, if you try to click it and then go in the research tree, there will be a button glowing blue telling you what research you need to build the ship.

You need to build enough military labs (the naming depends on faction) and then unlock the specific cruiser research that you want. When that is complete, you will be able to build that type of cruiser.

Hope this helps!

Depends a lot on the race and map! Generally, starting with 2 civic labs and getting some basic planet population research is hard to go wrong with. 2 civic labs will also let you research ice/volcanic colonization if any of them are near your homeworld.

Edit: If you're playing Advent, definitely go with 2 civ labs first, that's when they get their culture centers and culture is a very big boost to your economy!