Yay, your first cosplay ever! No worries, I was lost with my first cosplay too. But I will try my best to help you. Now, I’m not sure how detailed you plan to be with your costume since this one is your first (and Jinx has quite a bit of detailing in her outift), but I will try to cover all the bases or at least the basics.
Firstly, I’d check out this really great reference guide for a Jinx cosplay. It goes into great detail of what her entire outfit looks like.
For her wig i would suggest these two. Wig 1, Wig 2. Both are good long blue wigs and both of the sellers are good. All you would have to do is do two braids and tie them off with clear hair elastics. (make sure to bring extra with you because with so much hair to hold, the elastics tend to pop)