I'm a newbie myself so I may be wrong on this but I consider these newbie mistakes that I've seen in You-tube Let's plays...
1) Choosing a Conventional Start and not making the other required changes. On a player's first play-through this is tempting but it actually puts the player at a disadvantage unless certain other options are set right. Since this may put you behind other races, (NPR's), one should probably (at a bare minimum) bump up the # of research labs to allow for catching up.
1a) Electing to eliminate ICBM's in a conventional start and then worrying about required protection levels.
2) Being bothered by the frequent interrupts when a vessels gets too far from its home world and adding HUGE fuel tanks to the NextGen ships to compensate. Changing the conditional refuel orders to a higher level would have been a LOT more sensible.
3) Concentrating too early on strip mining the Sol system. Yes, Earth will run out of resources. Space must be mined but... hurrying to get mass drivers and auto-mines out to distant bodies when Earth's mines still outproduce the other bodies and Earth still has years of reserves of everything at better overall availabilities can be a mistake.
4) Giving a Sorium Harvester orders to motor back to a colony itself to dump 90% of fuel as conditional order 1 then having conditional order 2 be Refuel when under 20%
5) Firing researchers with poor bonuses in hopes that new and better ones will spawn more quickly.
6) Deleting an (empty) Task group while still high on battle Adrenalin. Turns out it had 6 ground assault units and their escort and...
If I'm wrong on any of these, please correct me. I'm always anxious to learn.
If I can't be a shining example at least I can be a glaring warning