This will come over time. You get credits the same way you get them in the campaign; by killing the enemies. As you progress through the different Escalation maps you will be able to purchase new weapons, TECH items, health and ammo. There are also stations that will ask you to deposit a certain amount of credits, these can open doors to new areas, provide extra defense and also upgrade the entire teams weapons. This might require some grinding, but shouldn't take too long to get. There is also an achievement tracker letting you where you currently stand.
NOTE: Before you finish your 15th wave on every map, spend all the remaining credits you have available on the weapon upgrade station, before completing the final wave.