You signed up for derby!! That’s awesome, go you!
People tend to have loads of different preferences when it comes to clothes/equipment. It depends on your situation but I think it’s good to hold back and try out as much as you can before you buy. If it is possible to borrow kit when you start, I think that’s the best way. You can ask what is popular in your league and what works with your floor skatewise. Additionally most leagues have affiliations with skate stores so you can find out how to get the best discounts.
My personal suggestion for a starting skater would be to get Riedell R3s once they are sure they want to commit, and also a good pair of knee pads!
I’ve seen people wearing kind of lycra sports skirts over leggings, but I just got a regular skirt, since I wanted a super swishy one, not the tight sporting kind. Most probably the internet is the way to go if you’re looking for the former.
I hope this is helpful!