Be sure you know your train number. If you are leaving from a major station, there will be a first class lounge and/or waiting area for sleeper passengers. The lounge attendant will call your train when it is ready to board, and lead the sleeper passengers to the train. There are signs, similar to the departure signs at airports, which will list your train and what track it will be leaving on, and when it is ready to board. (The signs work for everyone, coach as well as sleeper passengers.) There will generally be PA announcements as well.
Along side the train, there will be a conductor or attendant who will check your ticket (either a paper ticket or an e-ticket on your phone, tablet, etc.) They will tell you if you are at the right or wrong train, and which car to board. Be sure to get to the platform with enough time to go back and find the right train if you are at the wrong one!
One advantage of trains over airplanes is you don't have to be at the station 2 or 3 hours early. Twenty minutes is plenty, but only if you are familiar with the station layout and there is no danger of getting stuck in a traffic jam on your way to the station. For a first-timer, I would allow at least an hour.
The sleeper lounge (Metropolitan Lounge in many cities, Acela Lounge on the NEC) will usually have coffee, tea, cold drinks and snacks available, with comfy chairs, WiFi, etc. so a good place to wait if you are too early. (The lounge in Chicago even has showers, though they may be closed due to Covid-19.)
The first time I took a long distance train (LSL BOS->CHI in a roomette), I didn't know about the lounge and had to hustle all my luggage through South Station among all the commuters and NEC passengers, and find the right platform (which was well-marked) but once I reached the train, they took really good care of me. The lounge attendant would have guided me to the right track and arranged a Red Cap to bring me and my luggage to the train if I had needed that service. The Sleeping Car Attendant (SCA) always asks if you are a first-time sleeping car passenger and gives a very thorough introduction to the room, car and services if you are. They are always happy to answer any questions, even for experienced passengers.