Your best bet is to watch some shows to learn more about the discipline, and find a trainer to help you get started.
A western pleasure horse is bred to have the appropriate walk, jog & lope, and proper training ads finesse to those gaits for the show ring. A good trainer will be honest about whether or not your current horse will be competitive at the level you are interested in showing.
Show clothes and tack and can be purchased at tack stores in person or from the many online stores. Ebay
can be a good source, as well as tons of Facebook groups and occasionally even Craigslist.