For spin class, comfort and breathability come first. Wear moisture-wicking athletic tops and bottoms (leggings, bike shorts, or fitted shorts). Avoid loose pants that could catch on pedals. Sports bras with good support make a big difference for women.
Sneakers work great for beginners — no need to buy cycling shoes unless you want them later. If you do choose cycling shoes, SPD-compatible cleats are usually the standard.
A towel, water bottle, and optional gel seat cover can help you feel more comfortable as you build saddle tolerance.
Short version: fitted, breathable clothing and supportive shoes — and you’re good to go.