Database schema, how to learn to draw them?

Question

block diagram

Hello everyone, in general, the beginning of the school year, and here is a new subject Information Systems. The teacher doesn’t give a damn, she goes on business during tapes, the one that the whole group plays cs. .Don't understand how and where to start? Leave some examples of such schemes, or tell me what you messed up here, I drew such a scheme.
to this task.
You work for a commercial telephone company.
The company provides subscribers with telephone lines for long-
distance calls. Your task is to track
the cost of long-distance telephone conversations. The subscribers
of the company are legal entities that have a telephone point,
TIN, bank account. The cost of negotiations depends on the city to
which the call is made, and the time of day (day, night). Each
subscriber's call is automatically recorded in the database. At the same time , the
city, date, duration of the conversation and time of day are remembered.

Answer 1

Google UML. From open source software there is Dia.