Build rooms in your bunker to earn money. You can then upgrade them to make more money and build new rooms. Because upgrading costs quite a bit, at the start concentrate on unlocking all the rooms first, then upgrade them later to get extra stars. You usually have get a room to level 25 before unlocking the next room, however this will eventually change with mutant bonuses. Get as many rooms as possible to level 25 (and earn the first star), then work on the more expensive ones at the same time as you progress with the cheap rooms to get them to level 50.