This is a list of general principles that every newbie needs to know. I'm counting on the community here to help me flesh it out.  Please contribute!  I'll check in once a day or so and update the thread with new suggestions.

I'll start with my favorite:

1.  Don't die!  Three very bad things happen when you die: 

• The enemy gets anywhere from 1,000 to 3,000 gold
• The enemy gets a ton of XP
• You are unable to get gold, get XP or kill things for 30 seconds. 
  

The most senseless way to die is being killed by enemy towers while chasing a baddie.  Even if you think you have a kill, let the enemy run away.  The enemy loses out on 15 seconds of creep killing and you lose nothing.  That's a small advantage but it's better than the alternative:  an even trade of 2 dead demigods.