What passport to use traveling to USA with two citizenships?

Can I use my german passport to travel to USA or do I use my US passport?

And if I lived and worked in germany my whole life and never in the USA and never filed taxes can I enter USA without problems?

I just found out I am us citizen a few years ago don't ask why but I'm 34 and recently traveled to US with my german passport and they were all like "wtf where is your passport" and I'm like: "Here.." and they "Yeah your american passport?!" and I'm "I'm german" and he is like: "Dude you are born on american ground you are american!"

Well they let me travel in to the states without american passport but told me to go to the embassy as soon as I arrive back in germany or else I won't be able to enter USA again.

That being said, I done that. Now I have my american passport. But do I show both passports or only US passport? And after doing my research I found out americans file taxes every year. I haven't done it the last 18 years of working. Should I just not file? I will never work in the USA and I will never live in the USA. Or will I get problems at the airport? Can they see I don't file?