ARE THEY OUT THERE
With the infinitely expanding universe full of billions of stars and over 18,000 discovered planets, its actually very unlikely that Alien life isn't out there, somewhere. Whether they be intelligent, humanoid, Alien killers, or piles of sentient, Alien substance, the possibility of them existing is definitely great.
WHAT WOULD THEY LOOK LIKE
Alien life would look like their planet has sculpted them to look, just like we have evolved and adapted to survive on earth, and its possible that Aliens who live on a planet with the same climate and atmosphere as earth to look exactly like us. Although, given the same scenario, but have the Aliens on the earth-like planet be more evolved than us, their eyes and brain would at least be tripled in size, meaning they would have a much larger cranium, and their skin will be much thicker and pigmented. But unlike most Alien sightings, which describe them as small, they would actually be large. Evolution would not make them smaller, they would be about 8-10 feet on average. We will look like this in about 100,000 years.
Aliens could also be so different that humans cannot even comprehend what they are. Like clouds of space dust that communicate by changing the temperature around them.

Aliens on planets with heavy gravity would most likely be short and stocky creatures with an immense amount of strength to withstand the pull, water planets would have creatures similar to our deep ocean life, and planets with barely any gravity could have Aliens that float effortlessly.

HOW ADVANCED WOULD THEY BE
Our first contact with Alien life will most likely be with an unintelligent specie, due to our signals into space only being able to go so far, hardly far enough. Hopefully the Aliens will not be more advanced than us and hostile, in that case they would easily kill us off with their superior weapons, give us Alien diseases, and take Earths natural recources. Much like the more advanced explorers killing off the Native Americans and taking their land. Their is a great chance that superior carnivorous Aliens would eat humans. It is also preferable that we go to Aliens, instead of them coming to us, because if they are able to do this, they are far more advanced than us. It is also likely that Aliens know we exist, but we are so much less advanced than them that they do not want to waste their precious time with us.
WHERE DID WE COME FROM
When we sent the rover to Mars, millions of bacteria from Earth on the machine that adapted and survived are now living on the planet. As it keeps reproducing and surviving it will evolve. It will take millions of years, but we may have accidentally started life on Mars. Astonishingly enough, the same thing could have happened billions of years ago on Earth. Ancient Aliens or an Ancient Alien object could have transferred the single-celled organisms that started life on Earth. Maybe they did it accidently, maybe they did it on purpose. After all, Humans have 40% stardust atoms in our body. The same Aliens that dropped us off here could have done it for a purpose, and might still be watching us.
THEORIES
Alien sightings in which the Aliens have large eyes, and craniums, with pale skin, could actually be much more evolved Humans from the future who discovered a way to travel back through time.
Aliens could be sending us the same message every day, but their language is so different than ours, that we dismiss it as radio static.
So much of the ocean is undiscovered that there could be intelligent creatures living on the ocean floor.
Aliens could co-exist with us on Earth, but they are too different for humans to recognize
Its likely that Alien life wouldn't be carbon-based