The traditional training-wheels class is, of course, the Fighter. It's also the most popular, most relatable heroic-fantasy character concept.
Another obvious option (the better option back in 3.x, for sheer build simplicity, as well) is the Barbarian. Also super-familiar (thanks, Conan), but maybe not quite as relatable, with a bit of 'big dumb' baggage.
Both have the advantage of delaying sub-class until 3rd, so you don't even need too clear an idea what you want from the character when you start. (Wizard, in contrast, you get to deal with the baroque flourishes of neo-Vancian casting /and/ the features of your Tradition, from day 1.) Both have the disadvantage of not really teaching you the whole system. When you're bored with your fighter or barbarian, you're prettymuch starting over from square one, anyway.
Fighter -> EK could be a good introductory path. Start with a fairly durable, forgiving chassis, but, after you've got the d20 vs d12 and such down, pick up a little casting. Once you're ready to move on, you'll have a basic foundation in how most 5e classes actually work, and in the important resource-management game, both daily & short-rest.