Depends on where you live and who you talk to. According to the United States constitution, you are entitled to believe whatever you want to believe, be it in Judaism, Islam, Buddhism, Hinduism, Christianity, agnosticism or atheism. In some places, though, you can be jailed or even killed for being an atheist.
Yes. It is okay to be an atheist. It is simply a lack of belief in a deity. Regardless of what people may say, atheists don't want to kill babies or have an excuse to sin. We're no more immoral on average than any other group of people (and looking at prison populations in the USA, we're actually more moral than most groups).
As stated before, it is no more than a lack of belief in a deity. Every single other thing is a personal belief. Most atheists would tend to agree with scientists and most also agree with the tenets of secular humanism, but neither is required.
Generally, atheists are atheists because of a lack of evidence. It's the same as to why you don't believe in unicorns, sasquatch, santa, or the tooth fairy. There's no reason to believe in them because there is no evidence. The same is true for every deity ever created, but the god of The Bible or Allah of the Quran are especially bad. At least when people were claiming Zeus was real, there was actually lightning to see and thunder to hear. Belief in god or Allah is solely on faith.
Most people only believe in a single god and ignore all others. This makes them atheists to everyone who's god they don't believe in. We simply go one god further.
Most Christians believe that if you are anything but a Christian, you will spend eternity in separation from God, the state of which is commonly known as Hell or Sheol. As a result they feel very strongly about atheism and will try to convert you to their religion "for your own good". Be polite when you refuse but also make certain you are treated with the same respect.
Similarly, Muslims believe that atheism, or denial of the existence of god, is apostasy or blasphemy, and punishable by death (men only) or life imprisonment. Thankfully, the death sentence is rare, but prison is not great, either.
Atheists will tell you that they don't believe in myths and Fairy Tales, as that how they see religion. People of all different religions and contradictory beliefs are willing to die for these myths and fairy tales.
If Atheism is what you believe, then that is what you believe. You are entitled to that belief. And most people will not discredit you for that belief.
Its perfectly fine to be athiest, but many people don't accept it. I don't like to talk about spiritual beliefs with most people because, in my experience, many people look at you differently afterward. I only tell people I'm close to because they liked me before and they all still like me after.
Many feel that atheists are filled with hate for god and religion. In actuality atheists do not hate "god" any more than they hate the tooth fairy or the monster under the bed once they realize that they do not exist. In the same way "heaven" vanishes in the same way that Santa's workshop and the pot of gold at the end of the rainbow do when the truth is known