Which character set to use?
tl;dr: In most cases, you should use UTF-8 (or UTF-16).
Without getting too technical, a character set encoding is a standard for how computers process characters of written text. There are multiple standards, such as ISO 8859-1, but most of them are suitable for only a small subset of languages. Usage of UTF-8 is highly recommended, as it contains enough characters to represent almost any text written in most common languages. The technical reasons to use an encoding other than UTF-8 have become obsolete; e.g., memory is not as critical a factor as it used to be, now that computers and smartphones have much more memory than they did in the early days of computing.
Please note that some file formats, such as .txt-files, do not specify character set encoding, so the recipient of the file has to guess the correct character set encoding to be able to read the content. Therefore, it is highly recommended that you use only UTF-8 with such file formats, or even avoid them completely in favor of those which properly specify character set encoding and are always readable.