Congratulations on the steak! It can be daunting if you have never cooked one before.

While I am not one to tell another man what he should do with his own meat, I can say that a marinade is usually not my first option for getting the steak tender.

First off, it starts with the best marbled steak you can afford. There are plenty of "choice" cuts out there that fit the bill and you don't need to pay top dollar. The extra fats are a good start for the next step.

Reverse sear. Oven bake the steaks at a low temperature for as long as possible. Since most ovens only will go down to 200°F or so, that is what we'll work with. (Ideally you want lower, but 200 is fine.) The longer you can cook a steak, the better: The extra time at low heat will break down connective tissues really well, even with steaks that you think might end up tough. (Sous vide is an option and you can keep a steak at the proper temp for so long it turns into mush if you wanted.)

After the steak is up to temp, throw that bitch on a smoking hot cast iron pan for just long enough to develop a crust. You should be good after that.

Again, you do you. I prefer no marinades as I just like a steak that is well cooked instead of hiding it behind sauces and such. My main point is that you have options for getting a steak tender, chemical or otherwise. (Salt and pepper should enhance the steak, not hide its flavor.)