This will come naturally if you follow the initial tutorial.
Crabs wander around near the shore in the shallow water. Once you have crafted you first stone tool you can head over to find one. A few simple mouse clicks to kill it, then hold E to skin it.
You will need to craft a campfire (again this is part of the tutorial) by using Sticks. Then light it with kindling (2 sticks combined). Finally select your crab meat from your inventory and hold E to attach it to the campfire and the achievement is yours.
Hope this helps.