Fitness counts for more in 11 a side, outdoor football, especially junior football, but even so skill and ball practice is primary. A bunch of unfit, elderly ex-footballers will humiliate a super-fit bunch of newbies every time - I speak from personal humiliation.
The fittest players usually go into midfield, because they have to support the attack and come back to defend, and they have to be able to pass. Strikers need speed, guile and ball control. Defenders need height, strength and tackling ability.
If a ball is knocked off the side of an outdoor pitch, it is thrown in by the oppossing team. If it is knocked off the ends then either the goalkeeper kicks it out, or the opposing team gets a corner. A corner is kicked from the corner, normally straight into the goalbox, and normally most people there jump up to try and head it into or away from the goal.
As for the rules, bear in mind the rules are something enforced by the referee and two assistants, and are not something players live by. The only rule in life is don’t get caught. It used to be common that as players were about to jump in the box, an opposing player would grab their scrotum, so as to make jumping ill-advised. You would be red-carded (sent off) for that offence, but it is difficult to spot in a mellee, as is an elbow to the face when players are running side by side.
A dangerous tackle, where you raise your boots or obviously try to damage someone will always be red-carded, but less dangerous behaviour gets a warning ‘yellow card’, two of which equate to a red card. However, when a tackle successfully makes contact with the ball before making contact with another player, the followup collision is generally ignored, no matter how damaging or deliberate.
When a striker breaks away from the defence and heads to score, a defender will often pole-axe the striker in wht is called a ‘professional foul’. If they are the last defender, they will be red-carded. If any foul is committed in a teams penalty box by that team then the other team will get a penalty kick, which usually produces a goal.
The rest of the rules you will pick up in your first match, except the offside rule which you will find tens of thousands of explanations of when you play a full 90 minute match.
For outdoors, grass but especially astroturf, buy a cheap pair of trainers with molded studs. If you keep playing, you will buy better boots but they are good enough for six months to see how good you are and how committed. You get similarly cheap indoor boots, anything that sticks your to the floor and a bit of padding for when - not if - someone kicks your foot. My best friend has been on crutches twice in the last six months with different broken foot bones. Also buy shin pads that go under your sock, it’s the soccer equivalent of wearing helmets in American Football.