Why should one be moral without a god or religion?

Well though I might not be a very moral person, but when I think about why I should be moral, the only reason I can think of is because of what I read in the bible. I can’t think of why I would have followed moral guidence like good will for others,compassion for the less fortunate,honesty, etc.. specially that they are not punishable by law if there is no higher power that will rewards me for my good deeds in return, what would there be in it for me in return?

I would like to understand on what the moral ethics of atheists are based on mostly?