It is unfortunately rather common for less experienced Christians to say that Atheism is a religion. This is false, but it's not too far from the truth. In reality, atheism is a religious orientation, just like Theism. Just because one is a theist doesn't mean they're a Christian, because theism is a blanket term that describes any belief in any amount of divine beings above zero. Likewise, atheism is a blanket term describing a host of religions that specifically deny, or at the very least ignore, the existence of a divine being.
When someone says atheism is a religion, it's common for atheists to respond with "that's like saying off is a channel". In reality, however, atheism is like a channel that denies it has a producer. In every scenario, the channels still exist even if the TV is unplugged, and all of them have producers. Atheism is like a channel that ignores this fact, instead claiming the script for every show they produce was written by smashing someone’s face against a keyboard.
Whether atheists admit it or not, they have religious beliefs, simply because the scope of religion covers every aspect of life. Do you have views about the origins of life? About its purpose? About how it should be lived? If your answer to any of these questions is "yes", you have a religious belief. If your answer to all of them is "no", you're just wrong.
For an atheist, the most common religion is Evolution. The two are not inextricably linked, but they are often found together. Thus, when most amateur Christian apologists say "atheism is a religion", they most likely have Evolution in mind. But regardless of whether that is your particular religion as an atheist, the fact is you do have one. To figure out what it is, simply ask what it is you have replaced God with. What do you believe gave us life? What, if anything, do you believe is its meaning? How should humans live their lives, and what determines that? Answering these questions will tell you what your religion is.