For "aliens" (meaning from another place, unlike, different) to travel far enough and fast enough to meet us, there are some necessities:
They must be intelligent. Otherwise why would they be interested in coming? And they must be able to pass on knowledge to others at same lifetime and into following generations. Doing that must mean language. Even our own dolphins and whales and some other species have a language. Would it be by sound, signing, writing only? Seems doubtful that telepathy exists for that, no matter how far the brains have developed.
They must have sense organs to perceive around them. Eyes, ears, touch sensitive surfaces would do that for them. Arrangements might vary widely, no prediction there.
They have to have extensions of their body that can use tools easily. This would let them manipulate an environment to create a civilization.
They must have locomotion--a way to move from place to place under their own power.
They must be terrestrial (living on a land area) and not nautical (no mermaids or mermen). Water creatures cannot manipulate energy like fire in order to develop metals and control chemical reactions in contained areas.
They might be non-violent, and just curious. Or they could be interested in resources that we have, and consider us just in the way. The second idea is very doubtful. There must be lots of other places and easier ways to get stuff without going to war.
The chance of their being biological beings like us is low. That is because organics have a limited duration in the timeline of the universe, galaxies, stars, planetary systems, and evolution. A much greater chance is that the "aliens" would be technological or electromechanical (robotics) instead. Longer existence, easier development to greater capabilities, ability to endure more extreme conditions with less energy used, less volume of a spacecraft per traveler, multifunctions of one unit possible.
But all is just speculation since we have as yet not a single other indication for any life at all anywhere. The jump to considering what "aliens" might be like is just too wide or high or broad (all three dimensions, get it?)