Scrapyard
Once you have stolen a car you can opt to melt it in exchange for bullets. Each car has its own melting value that is also influenced by the damage it has, these can be changed on the repair page.
If you are part of a crew you may also receive a melting reward bonus for hitting crew boss determined interval or milestone. Your crew will take a set % of all bullets you melt.
Click the button highlighted with the Blue circle for which car you'd like to Melt and then hit Melt Car highlighted in Red. Simples!
Owning a Scrapyard
If you own the scrapyard you will be able to control prices/stock here also, adjusting price or listing the scrapyard for sale.
You can also fill the scrapyard up with more stock as the people melting in your location will contribute to the amount of bullets you have.