The fifth mission of the campaign will be your first encounter with the flood. Common flood fighting tactics include (please post more in comments):
--Shotgun or similar close range weapon to deal with charging flood. There is plenty of shotgun ammo in this level. Watch out for flood with shotguns.
--Assault rifle for clearing swarms of flood. These guys are hard to control. Kill them quick, before they can revive fallen foes. The plasma rifle has the advantage of making it easier to avoid reloading, but it's a worse choice against stronger flood.
--Battle rifle. The Flood can be headshot now. It takes some practice, but it kills them fast.
--Anything with fire is good against the flood. There's a flamethrower about a third through the level.
--power weapons, including plasma grenades. Do NOT use plasma grenades on charging flood! Obviously, use any powerful weapons you find. Saving them, and your plasma grenades for the Pure Forms (Hunter Flood) towards the end of the level is recommended.
The last third of the level (after the cutscene with the elite drop troops) can be ran through quickly. Once you enter the crashed ship, you're (mostly) safe from harm.