Do your side missions, encounters etc to try and get all three areas in the Base of Operations to 100%. For the main missions, invite people for them as it really helps.
Weapon and gear drops - deconstruct 80% of what you get and sell the other 20%. Deconstructing will help with crafting weapons later on in the game, and selling will help for rerolling stats on gear as that requires credits.
Obviously if you find items that are better than what you have - keep them and deconstruct or sell the older gear
That will get you a good amount of credits and parts for crafting.
In addition, add some people off here and we can run your main missions together to help you through parts