Your objective is, not surprisingly, to destroy the Ork base. The resistance gets a bit heavier, though.
Go check your two defense outposts and capture both SP's your men are standing around. Finish the turrets you got, build 1-3 around the other SP as well. For each outpost, you want a heavy bolter squad, a missile squad, one of your commanders, the Imperial Guard squads you started with, and the turrets (1-2 upgraded with missile launchers). Get an Armoury etc. accordingly and set this up. The Orks will make annoying attacks with Slugga/Shoota Boyz and Wartraks, but said defense will be sufficient. Keep the Guardsmen back, they're not made for absorbing damage.
When your defense is set, you have some options. The Orks have Da Boyz Huts on the left and right, and a Da Mek Shop in the center. If you go right, you can salvage a Machine Cult and get Dreadnoughts, but you don't need it right away. I would go for the Da Mek Shop first to cut down the amount of Wartrak annoyance (some will still come from the main base, though). To achieve this, take 1 missile squad and 1-2 anti-infantry squads. Scout with infiltrated Skull Probes and missile the buildings from afar. Repeat on your left and right. You're done.
Next there's the three big holes in the city wall with Orks coming through. Get all SP's, build fortified LP's, move some infantry over and build a couple of turrets around each LP if you want to. Then go through to destroy their main base. There are Da Mek Shops at both corners; the eastern one is weaker (their Settlement being at the west corner) so you may as well start there. Send an infiltrated Skull Probe to check out the whole camp in advance.
Build 3 Dreadnoughts (no ranged weapons) set to an attack stance (F1, F3 or F5) and follow them with 1 missile squad and 1-2 anti-infantry squads. Let the Dreadnoughts loose and support with Marines (missile squad kills any Wartraks first and buildings second). Take the east corner first and then move west - ignore the center, it only has Waaagh! Banners you don't need to destroy. When the Orks are trying to do something about your attack, you can move in a couple of squads from the other direction and start destroying their buildings from behind.
When the main structures are down, you win the mission. Remaining Waaagh! Banners don't count, ignore them. You will proceed to the next mission, Destroy the Xenos.