First, set up your base quickly and get the ODST research from your Barracks ASAP, as well as the MAC Cannon research from the Field Armory. Note that there are two Forerunner Supply Elevators (basically Supply Pads) to the left and right of your base. Send one troop of marines to each to increase your resources even more quickly. You'll come across three Scarabs on this map which you can ignore completely (unless they go after you instead of the Flood, in which case the MAC Cannon will come in handy).