Thickness
A thinner string (1.10–1.20 mm) gives more control, feedback and “bite” on the ball, but breaks sooner. A thicker one (1.25–1.30 mm) lasts longer and is kinder to your wallet, but with slightly less feel. Controllers and advanced players more often choose thin strings, beginners go thicker for longevity.
Tension
Lower tension (10–11 kg) — more power and “trampoline”, the ball flies on its own. Higher (12–13 kg) — stiffer, more accurate, but demands your own arm. A middle reference for squash is 11–12 kg. Many manufacturers print the recommended range right on the frame.
How often to change
A string loses its properties not only when it breaks, but also over time — it “dies” and goes dead and wooden. If you play regularly, restring every 1–2 months even if it is intact. Fresh strings are like a new racket for little money.
Do not know where to start? Take medium-thickness strings from your racket's brand and a tension from the middle of the recommended range. Then adjust to feel.