There is an old saying that 'the amount of times you play a week, is the amount of times you should get your racket restrung a year'. This is a good baseline to follow but as stringing technology gets better, that rule is heading towards double, meaning if you play once a week, you should get it restrung twice a year, once every 6 months.
This is personal preference but keep in mind that the looser the strings, the more power you will get and the tighter the strings, the more control. Check your racket frame, it should have the range of string tension. The coaches will save a note of your set up so if you like what you had the time before, we can match it.
A polyester string is very firm and loses tension rapidly meaning it will give you the most problems with your elbow. A multifilament is a lot more forgiving on the elbow as it absorbs the shock better thanks to all the tiny fibres in the core, however this doesn't get you much spin. If you are looking for something forgiving but help with spin then a hybrid might be the way to go.
