Honestly any gun you can get your hands on. Though, many will say shooting a .22lr rifle is the way to start and I’d be inclined to agree with that. I learned on an AR-7, and like many people who started shooting young, I think if I started with something bigger I wouldn’t have enjoyed it as much.

Ruger 10/22s can be found relatively cheaply and serve as really good starter guns. You can also easily outfit them with aftermarket parts and make them a mainstay in your collection after you expand. You could also get a Cricket, bolt actions can teach new shooters a lot about proper shooting form and the importance of hitting the target without wasting much ammunition. Also, if in the future you end up teaching new shooters, having a bolt action helps a lot with safety. It’s a lot easier to tell if a bolt action is safe, than a closed-bolt automatic.

Generally speaking, new shooters - especially ones who are scared of it - will enjoy the smaller calibres at first. So, it’s arguably better to find one that doesn’t seem scary to you while you’re learning to get used to it.