Measure to find out whether your guitar is a full-sized guitar or a shorter-scale guitar. On a full-sized guitar, the length of the strings, from the bridge saddle to the nut, is 648mm.
For a beginner, if you have a full-scale guitar, just use a package of nylon classical guitar strings labeled "standard tension" or "normal tension". Any brand will do.
The numbers indicate the diameter of each string, in fractions of an English inch, or in millimeters. Some musicians prefer a particular diameter of string for each of the six string positions, so they want to know the exact specifications.
If you have a shorter-scale guitar, you may need to find a special set of strings (all of which will be thicker in diameter) designed to provide the proper tension and tuning for a short-scale instrument.