They'll look like robots.
It seems to me that the most likely method for first contact will be through an artificial intelligence. Our own best chance (at this point in history) for making first contact, to teach others about we humans, is through our space probes. Voyager 1 is currently more than 18 light-hours from Earth. Attached the Voyager probes are Golden Records, bringing a message from humanity to any space faring races who may come across them.
The vastness of space and the very solid theory that (effective or actual) faster than light travel is not possible (or at least not feasible) means that actual biological aliens are very unlikely to visit us. Just as unlikely as we are to visit them.
Suppose they sent a generation ship or a cryo-sleeper ship. If they were coming here because they captured some radio waves or followed a probe back, they might know something about us and even what we look like. But because travel will take such long time, they couldn't even be sure we'd be the same when they arrived, or that we wouldn't have destroyed ourselves in the intervening decades/centuries.
This leaves non-biological emissaries. Robots. It's really the most logical method after sending a signal via light to let us know they're coming. The robots would come, hopefully with a measure of intelligence of their own. They would impart knowledge to us and ask nothing in return, as a benevolent race might do. Perhaps they could even bring some quantum entangled communication device, so that we may communicate with them in real time. It probably doesn't matter what they look like as long as we're just looking at a hologram. Modern humans can very easily accept what they're looking at if it's not in the room with them, we're used to seeing things that are not really there. This would be quite different for ancient humans, but then they wouldn't have a concept of benevolent aliens in the first place.