From Halopedia, the Halo wiki
When the level begins, you are prompted to build a Firebase. Simply highlight the lighted area and select Firebase from the building menu. Once you build the Firebase, Serina will tell you to build a Supply Pad which will produce resources to allow you to be able to produce more useful upgrades. Choose an empty building plot and select supply pad. Once again, you will be asked to build another building, a barracks. The barracks will allow you to train infantry units. After you build a barracks, you will be asked to train five marine squads. Simply select the barracks and highlight marine, and select it five times. After you train your squads, you will have the option to build structures on your own. You may want to upgrade to a fortress which will give you two more spaces to build on. Keep building more structures until you are comfortable with what you have. I do recommend building a Vehicle Depot, but after that it is up to you.
After you train a few infantry units and create a few Warthogs and Scorpions, you will be ready to do what the mission was for, proceeding toward the relic that Forge spotted. Select all of your units and follow the blue arrow. Once you get to the large field with Covenant guards, it's time to think strategically.
First of all, when you select all units, you should highlight the crashed Pelicans in the middle of the field and press X. Your marines will use them for cover, preventing them from being damaged. Use your vehicles to attack the sniper towers and finish off the ground troops when you are ready.
Once you take out all of the Covenant in the field, eject your marines from the cover by selecting the pelicans and choosing "Eject". Press up on the D pad and choose heal and repair from the circle menu. Try to get all of your units to enter the diameter of the green circle, so that you can heal your units and make them ready for the next battle.
If you lost some of your units, you can go back at any time and train more units. Make sure your army is at the maximum capacity, or wherever you are comfortable at. Once you are ready, proceed up the field.
You will see a Covenant base up ahead of you, which is an optional objective if you wish to destroy it. If not, proceed down the road following the blue arrow.
There will be a shield barrier preventing you from accessing it without using marines. Enter it, blow up the shield generator, and get out.
Behind the shield there will be various Covenant units, including Ghosts. Use your marines grenade ability (Y) to destroy the ghosts, as well as your vehicles. Use Scorpions to destroy the sniper towers and take out all individual remaining units one by one. Once all of the Covenant units are down, proceed down the road to a large Detonator. Using all of your units, use their Y ability to blow it up and finish the mission off.
Once the Detonator is blown, your squad will proceed into the relic and investigate the Forerunner structure.