Squash is moderately simple.
Initial server decided by coin toss or racket spin or whatever method you prefer.
The server must serve with at least one foot in the serving box, but behind the forward line, the ball has to go above the higher line and land in the opponents half. After this, either player can hit the ball anywhere as long as it hits the far wall above the lower line. The server is the only person who can score a point - if the returner wins against the serve, they take up position as server. Service alternates sides while the same server serves. Then it's first to 9 points (or two clear points after that).
Very tiring game, and can play havoc with your knees. The best players will walk all over you without seeming to raise a sweat.
According to the wiki, I've posted the "English scoring system"
PS - don't hit the ceiling or have the ball hit above the high side & rear lines (lose the rally for that, similar to an "out" in tennis)
PPS - I forgot there was a top line on the front wall, too - stay below that :-)