I need to do my first strings change on my RBX 170 and I don't know which kind of strings I have to buy.
How can I understand which one are best for it?
Your bass has a 34" scale length, and shipped from the factory with roundwound strings fitted. If you like the feel of the strings, especially when the bass was new, then you simply need to find another set of long-scale roundwound strings.
The gauge of the strings fitted to four-string basses is most commonly 45-100 or 45-105. The numbers here reflect the diameter of the strings in thousandths of an inch. Manufacturers will report the scale-length their strings are made for.
In the above image, Rotosound report the length and gauge of their strings as an integral element of the string packet design. The numbers '66' and '88' are their own proprietary cataloguing/branding system. Under these numbers it says "long scale", which indicates they will fit a 34'' scale length instrument. The gauges of the individual strings are listed under the name of the strings.
There are numerous string manufacturers, with many different string types available. A good idea might be to work out which famous bassist you like the tone of, and then find out what strings they use. There isn't really a wrong answer here, other than that you might intensely dislike the feel and sound of a set of strings your bass hero swears by.