Get a pattern and slowly as you get better, adapt and change your pattern, such as neckline, skirt fullness and length, sleeves or no sleeves, etc. Take plenty of time cutting your pattern and make sure to use sharp, sewing scissors. If you are unsure about the fit, air on the side of a slightly larger pattern and then sew the dress tighter as you sew and fit the dress to your skater.
Find some patterns using these links or by visiting your local fabric store (like JoAnn): https://jalie.com/collections/sport-patterns-skatewear, and Kwik Sew 3502
Always use 4-way stretch fabric on the bodice of the dress and it is recommended that you use 4-way stretch fabric on the skirt as well. What this means is that there is stretch when you pull the fabric up-and-down and left-and-right. You can get fabric at a general fabric store, or for more colors and options, try these online stores: https://spandexworld.com/ and https://www.spandexhouse.com/
Use all-purpose thread and typically use a zig-zag stitch on your machine. You can also use a "stretchy" needle, or ballpoint needle, to help sew the 4-way-stretch fabric to keep it from "running away". A serger can also help with elastic insertion, skirt ends, etc., but it is not necessary.
Now comes the fun part. Once you have made your dress and fitted it to your skater's body with no bunches, etc., you can start adding crystals. I recommend purchasing Swarovski flatback crystals and gluing them to the dress individually using either E6000 (stronger hold, toxic fumes) or Gem-Tac (water based, slightly weaker hold, non-toxic). The most typical crystal to buy is a round flatback in ss12, ss16, or ss20 (these are crystal sizes). You can choose different colors and shapes. A go-to color is the classic "clear, white" crystal. These crystals go with everything and are the most sparkly. You can buy crystals at craft stores or at https://www.rhinestonestore.com/Swarovski-Flatback-rhinestones-the-Rhinestone-Store-s/3739.htm.