Mission 01 - Alpha Base
Fight your way to Alpha Base.
It's something like training mission. Get used to the controls and follow the instructions on the screen. Then just go down the road and follow the arrow - eliminating any enemies you encounter.
Use Marines to Destroy Barrier Power Source.
Only the infantry is able to move pass the barriers - send some squads to destroy the energy generator. Don't forget about Marines special attack - the grenades (Y).
Eliminate Covenant within Alpha Base.
Well - just get inside with all your forces and get rid of the enemy.
Find and Rescue Pinned Marines (4).
This objective is going to just finish itself - just eliminate enemy forces you encounter and all the marines in trouble would join you.
Kill 100 Grunts.
Just kill EVERY Grunt you encounter.
How to get the skull?
Just before the Alpha Base, near the downed Banshee.
Required objective: Kill 100 Grunts .
Watch out - near the end of the mission, after you kill 100 Grunts - leave some enemy units alive. Instead - mission would end. So - leave them for a while and get the skull. Then - finish them.
Under the bridge near the Alpha Base.