First, get a train. Although you can get everything separately, safest bet is a startet kit because it comes with the train, rails and power device. Then, even if your goal is a full-room layout, start by doing small dioramas or modules, because otherwise you will get overwhelmed or the end result will never satisfy you. Also, if you make the modules following the pattern of your track, you'll have a layout anyway. So that's it. Youtube is filled with tutorials and layout videos to learn from and get inspired.
Like any hobby, this is a long-term effort, so just enjoy the ride.