How can I mark that I have already seen a movie or an episode?
Hi, and welcome to TMDb, Xristina!
If you rate things you watch, they'll then appear listed on the "Ratings & Reviews" place on your TMDb member home page. (I personally normally only rate stuff I deem worthy of 6 stars or higher, so as not to clutter up my Ratings & Reviews list with films, series, etc. I determined to be duds that I'll never want reminder of.) Then, too, when you go to the overview page for a particular movie, show, episode, etc., the rating star for whatever it is will appear in yellow, which indicates that you've rated (thus watched) it.
You probably already have noticed, but though the TMDb rating system shows 5 stars, each of those stars has halves, thus it's actually a 10-stars rating scale.
Yes, it's usual in a movie review website that the movie you have rated will appear in your website. So, when you wish to review a movie or an episode, just go to your profile and check which are those you have already rated.