Interesting question. If you look at the number of stars on the sky some night and count them, you'll get the little bit of how many star systems that can actually exist in the universe. It's that big, and even bigger. We don't even know where the universe begins or ends, may not even have such a thing.
Now if you consider a very small percentage, like 0.0000000000001 percent of the universe for example, that life may exist on another planet (after all, life exist here on Earth!), now multiply that percent with infinity (how big the universe is) and what will you get? Infinity is the answer. That's the simple calculation that basically says that if there's even a 1 per a billion chance that another planet can have life, the size of the universe is so big that even that small percentage becomes significant, in fact there will be infinite cases of other planets having life. It's just because the universe is so big, and the distance between planets are huge, we don't often see or speak with life forms from other planets - we simply don't have the technology to go that far that fast.
So yes I do believe we're not the only guys here in the universe. But an alien life form may have an entirely different way of life than us. Their environment may be intolerable to us. They may have genetic materials (DNA, RNA for Earthly equivalents) made up of 20 different bases (rather than just A,T(U),C, and G) and linked together by backbone made of Arsenic instead of Phosphorus. Who knows? We simply is too small compared to the universe.
