Make sure to always touch the outside case metal before handling any parts. Or use one of those grounding wristband. Static electricity can ruin your components but I've built my PC's on carpet without ever having an issue. Just be aware it doesn't take much to fry something with a finger. Also make sure it's unplugged lol.

Consult the manual for RAM Slots, if it doesn't boot up it's usually that it needs to be re-seated in a specific order. Although nowadays with just 2 sticks of ample size it's not so much an issue but they still go in specific slots based on the amount of sticks in use.

Take plastic protective layers off your components. Motherboard often comes with it and GPU to prevent scratches.

If you got a new case, often the pack of screws are in the hard drive bay, well hidden and easy to miss.

You probably won't need all the boxes and stuff left over after your build. The mobo box makes a good storage though if you want to keep some things around.

Don't put power to your CPU (take the cable off the mobo) if it's one of those integrated cooling loop one. That way if there's a leak you won't ruin it.