Atheism is not a belief. Nor are Humanism and Secularism the same thing. Atheism doesn't even mean you are opposed to religion, though you may well be all of those things.
It's amazing how often I encounter people who say "I believe in god and you believe in Atheism. It's just a different belief".
Atheism is simply the lack of a belief in any gods. It is not a club you join, but an observation. It shouldn't even need a special term, but it's quicker to say than "I don't happen to believe in any gods".
You probably don't believe that there's a small furry creature from Alpha Centauri under your chair. That didn't require a decision on your part not to believe. You didn't sign up to a special "not believing in small furry creatures from Alpha Centauri under my chair" club. There just isn't any reason why you would think there was a small furry creature from Alpha Centauri under your chair.
An atheist is probably a Humanist too, but you can't assume that. The definitions of Humanism vary, but the British Humanist Association defines it like this:
A Humanist is someone who:
Trusts to the scientific method when it comes to understanding how the universe works and rejects the idea of the supernatural.
Makes their ethical decisions based on reason, empathy, and a concern for human beings and other sentient animals.
Believes that, in the absence of an afterlife and any discernible purpose to the universe, human beings can act to give their own lives meaning by seeking happiness in this life and helping others to do the same.
A Secularist is of the opinion that government and religion should be separate. That includes many religious people who consider their belief to be a personal thing and certainly don't want someone else's priests telling the government what to do.
Most atheists would be secularists too though not all of them.
It's important to remember that everyone is born an Atheist. As you grow up people whom you trust may tell you that a god exists and you may believe them or not. Which god is an accident of birth. In western countries you will typically be told that the Christian god is real, but in Islamic countries you will probably hear that Allah is the only god.
If you do decide to believe in a god then you get to pick which one is real. The fact that this is a choice and the kid next door can make another choice ought to be a bit of a giveaway if you are paying attention.