Reddit mentions of Sewing Activewear: How to make your own professional-looking athletic wear

I would start with something basic like a simple top and bottom before you get all fancy. Sewing with stretchy fabrics is a little different.

You will want to use a zigzag stitch or, if you have one, a serger for your seams.

Also you need to become familiar with the concept of "negative ease". Which is the garment is made actually slightly smaller than your measurements so that it is already stretched a bit when you are wearing it. If you buy a premade pattern this is generally already taken care of, but it's something you need to worry about if you make your own pattern.

When looking for stretch fabrics I would suggest looking for 4-way stretch fabrics. I would start with either an athletic brushed poly or a swim type spandex. Absolutely do not use stretch velvet for your first garment; It looks cool but is an absolute nightmare to work with.

I have found this is a good book to use as a reference.

I get a lot of my fabrics from SpandexWorld, Zenith & Quasar and Etsy has some great stuff too.

There is a Facebook group called Sew Aerialists for Polers and Aerialists who make their own stuff.

I've had good luck with patterns from Jalie for stretch stuff, although I've only used their leotard, unitard and leggings patterns.

I really like Johanna Lu of The Last Stitch channel and she has a book out called Sewing Activewear and a playlist for Sewing with Knits.