What’s that one invaluable skill you believe every tech newbie should learn? 
@nixCraft
1. Make backups and make sure they work.
2. RTFM.
3. RTFEM (read the fucking error message). If you don't understand the error message, GOTO 2.
4. If you get code online, take the time to understand it. This will help you learn and also prevent you from accidentally pasting malicious code.
5. Whatever a corporation says, assume they're either trying to sell you something or get your data, probably both.
Edit: Oops, just saw you said "one". Well, now you have five.