You can gather salvage by blowing up infected buildings, blowing up explosives/objects, collecting piles of salvage and killing enemies(After buying an upgrade). I recommend you buy Autopsy from the upgrade menu which allows you to get salvage for each enemy you kill as soon as possible. Piles of salvage give you 30-75 salvage each so check every corner for them. Please see the Salvager achievement for more info on these. You can easily get this achievement on your first playthrough if you check thoroughly for salvage piles and blow up infected buildings.
You also get a lot of salvage while playing the infestation game mode so don't worry if you don't get 25,000 after beating story mode. All the salvage you get from the infestation game mode can also be used to buy upgrades. To check your progress press > Extras > Player Stats > >Achievements and then find the achievement and it will have your current progress.