First I would like to know what are datacards. How do they work? Does it mean that the more you have the more powerful you are? I have noticed that some factions have many and some don't have any. Second, when and how do you get to know if you can use the force? Third, what do people mean when they tell you to reach them via darkness? I would appreciate it if someone answered, thank you.
Datacards are just what the faction can produce. Distribution is more or less random based on a variety of factors from when each faction was originally created and has nothing to do with power or wealth or anything of the sort.
To find out if you are force sensitive you need to be tested in game by someone who is. The best way to get in contact with someone is through your faction leader who may have some contacts. Force sensitives generally do not test brand new players since there is risk involved and since it is in extremely high demand, so you may be out of luck for some time.
This interface is nicknamed Darkness, so to message someone via Darkness you want to click the little envelope icon up in your icon bar at the top of the page, and that will take you to the built-in messaging called Darkness messaging.