But how am I going to choose my values?

That, to me, is the most important question. There are so many approaches to morality and so few good ways to choose from among them.

I think you have to have an end goal in mind before you can start computing which action is "better." But whether your guiding principle is "the greatest good for the greatest number" or "the most benefit to me"...there's very little to guide you there.

I was a Nietzsche fanboy in college, and the most significant thing I still retain from that is the idea that my end goal is to become the kind of person I wish to be. I create myself with every moral decision like a sculptor working clay. And while some defects can be hidden from the public, the sculptor knows where all of them are.

So how do you see yourself? What do you value? Consider this question the same way you would ask yourself what makes a great piece of art. Once you have this, you can adopt whatever moral formula moves you in the right direction. If you don't believe in universal moral truths, what else is there to start with but your own judgment?

I've found that I value people's trust, friendship, and respect; and I want to be someone who is worthy of it. I strongly dislike the feeling that I am deceiving someone or that I might be found out. Whenever I consider lying, I ask myself "is this the kind of person you want to be?" Sometimes I decide that the proper action for me is to lie (I also value being a person who provides for my family, so I might lie to keep my job...maybe), but I'm usually angry about being put in that situation - it's as if someone has made me add a defect to my sculpture.

Obviously, this is a highly personal approach and it's not suitable for holding other people accountable or managing an orderly society. It's the accumulation of a couple decades of life experience over dimly remembered philosophy. I hope it's at least useful as one data point.