So, being an ex-racer of 21 years, I have a bit of expertise here.
Let us start with leagues:
You will find yourself with two types of leagues, depending where you are.
You will have your local beer league which is much more relaxed and consists of all walks of ski life, including ex-racers, park skiers, mogul skiers, etc., as it is much more of a social scene than an actual race league. It is team-based normally, not individual based. Obviously you have some competitiveness, as there is usually a prize for the winning team, but it is much more about camaraderie, skiing on a week night, and just having fun.
Then you have your masters leagues. This is where racing is about the individual results, where spandex rules, wax matters, and people take it super seriously. I am not 100% sure about how training or anything works, but it is worth looking into at your local mountain. It generally involves traveling on weekends to other mountains and dedicating more time and energy than beer league. That being said, it is apparently very fun and a good time so long as you temper your expectations of results. But you will find ex-NCAA racers, FIS-level athletes, and USSA-level athletes who do take it super seriously.
Gear-wise, beer league requires almost nothing beyond what you ski on normally. Helmets are normally required, but that is it.
For Masters, I believe you are required to have skis that fit under the criteria. Here is a PDF from the USSA website detailing everything in regards to rules and regulations.
I imagine you will be required to have SL skis that are 165cm, and GS that are 180cm or larger. Boots are usually more relaxed, but if you win, they will almost certainly look to make sure you are within regulations.