I believe in only one thing: radical common sense.
I tried ways of defining myself but they were all too limiting.
We have built-in desires in our natures. We desire to live in a community, so we also have built-in morality, really. These are the two "sources".
Common sense builds upon the sources to decide what to do or how to respond to situations. We learn about our world through experience and communication and this is our map. The map works in terms of "if...then". If I do this, this will happen. Our values, connected via common sense to our sources, tell us whether the thing we predict wil happen is desireable.
The reason I call it RADICAL common sense is that I admit no haziness about it. Most people can't connect any of their decisions directly to their sources. For them, they learnt their values from others, and so they're so confused with conflicting good and bad advice that they're practically blind.
I decide that the input I get from others is so unreliable that I can only really trust my own experience to make a value system that connects with my sources.
ah, btw, I highly recommend stevepavlina.com for thoughts on these matters.