I tend to start with a setting or purpose myself, then go logically from one function to another until I have a basic outline of a race and society. Like so;

Start: I need a sapient race that's physically larger than humans and inhabits tropical islands.

-Large animals tend not to be native to tropical islands, so they have to be from somewhere else...

-but they can't arrive there by ship or anything, because they're too large to transport...

-and whales in tropical waters tend to be intelligent, large, and capable of travelling through water, making them perfect, but they're a bit too big...

-but isolation on islands sometimes leads to physical shrinkage to adapt to less resources...

-which means that they couldn't be able swim huge distances if I wanted them to be a reasonable size...

-so maybe they cannot stay underwater for too long (way smaller lungs) but still do work underwater...

-which means that they might build coastal traps for fish and crabs for food...

-which implies that their fins became crude manipulators as they adapted to living on land, though that says nothing about their flukes...

-which in turn implies that they still have them, making them more comfortable moving in water than on land...

-so their buildings probably would be simply adorned but perhaps complexly engineered, with a balance water to move in and air to breathe...

-and having precisely balance water and air in your building would be a great status symbol...

-which means that engineering, and in turn math and physics, are highly valued in their culture...

-which ultimately means that the race would favor both intelligence and practicality in individuals...

-so that hints at a somewhat meritocratic approach to governance...

-and when factored together with how whales form extended family groups in their pods in real life, you might get competing clans, based on a core family group but adopting promising individuals as full members, and on top having a clan where each member is another clan (a government) that works on similar methods.

I could go on forever, but so far we have a race that probably look like a cross between a whale and a mermaid, live on the coasts of tropical lands, practice aquaculture of various forms, tend towards practicality and judging others based on their merits, and rule themselves through an aristocratic council of some sort; a good outline for a civilization to write about or have others encounter.

The logic in these leaps doesn't have to be especially coherent, direct, or, well, logical (you can always plug the holes later, and the plugging, at least for me, is my favorite part), but they should make a little sense at least. Also, if you happen to like obscure trivia, here's where it shines; being able to link two ideas that do not belong together in nature is the best way to start an in-story conflict.

So, basically extend this for several pages, come up with reasons for anything I missed earlier, and that's how I create races.