As soon as it lands successfully in their party, it will evolve into Steelix. Jump into the Union Room with a friend, initiate a trade, and send Onix over to them. Make sure your trading partner is clear that you want to get your Pokémon back after it evolves: You wouldn’t want to trade with a random person and have them run off with the Steelix you worked so hard for. You’ll then need to find a friend to trade Pokémon with since Onix only evolves when it’s traded while holding the Metal Coat.
Upon receiving a Metal Coat, give it to Onix to hold. The only other reliable method to get a Metal Coat is to speak to gym leader Byron on Iron Island after you defeat the Pokémon League Champion and receive the National Pokédex. There’s a five-percent chance that a handful of wild Steel-types, including Beldum, Bronzor, Magnemite, Steelix, and Bronzong, will be holding one when caught. Once you’ve caught an Onix, you’ll need the Metal Coat item to evolve it. (If you don’t want to go through the trouble of evolving an Onix, Steelix can also be found in the wild on Iron Island and on Victory Road.) Later on, Onix can be found on Iron Island outside of Canalave City and inside of Victory Road on the way to the Elite Four.
It only has a 10-percent chance of appearing, so it may take some time before one appears. The earliest place to find one is in the Oreburgh Mine, which can be accessed just before the first gym. While Steelix can be caught in the wild, to get all the benefits of its evolutionary move set, you’ll need to start with an Onix.