Plasma is required to keep troops trained. If you run out of Plasma sell back any unneeded troops to get all of their Plasma back.
The Shield will protects your base from other players. To activate your Shield for free, leave your command center outside the walls and allow enemies to attack it. Then, let the shield activate to protect your resources.
Do not spend Ntrium when completing the tutorial. You can then gain free Ntrium by completing the tutorial.