Riding the Tokyo Metro can be an adventure for the newcomer. Here's one mistake I made.
Most stations have two tracks - one for each direction. The platform is either between the tracks on or either side of the tracks. In the latter case, there is sometimes a tunnel between the platforms so you can get from one side to the other. Some stations don't have this tunnel.
After going to a hobby shop near the Ginza line's Suehirochou station, I went back to the station through the same entrance I used before. Without thinking, I bough my ticket, passed through the wicket, and then realized that the trains arriving on the platform were going the wrong way. There was not way to get over to the other platform! I suppose I could have gone to the next large station and hoped there was a crossover. The better choice was to explain to the station agent what I had just done: "I made a mistake. I just bought this ticket but I want to go to Kanda". He let me back through the wicket and told me to go up the stairs cross the street and come down on the other side. On the other side I started to repeat my explanation to the agent there, but he just opened the wicket and waved me through before I could finish.
The lesson here is, when exiting a station think of where you want to go when you return. Do you want to keep going in the same direction? If so, go back through the same entrance. If you want to go back the way you came, look around for a metro sign on another corner, cross the street, and use that entrance.