Pretty much what @equestrianjess said. For the beginning you will be fine with jeans and your western boots, definitely a helmet. As you progress I would get a pair of riding breeches or tights and then a pair of riding boots (probably paddock boots and halfchaps first and then later on tall boots). Personally I don’t like wearing gloves because I feel like I can’t feel the reins enough, but it’s all personal preference. Hope this helps! Have fun riding English and hope it goes well
Just for starting out you don't need much, probably just a helmet. Jeans and your western boots could work for the first couple of lessons. I've seen many beginners take there first lessons in jeans and tennis shoes. They all wore a helmet though. But if you start to get more serious about it buy some breeches, English paddock boots and chaps, maybe some gloves because my hands are rather sensitive and they blister easily without them but it may be different for you.