Congrats and welcome to the world of refereeing!
Basic stuff you'll need is a kit (Shirt/shorts/socks). Nike is the best quality out there and you can pick up a set quite cheaply at many places. Also a pair of mainly black football boots.
You'll also need: Red/yellow cards, match record sheets to write goals/bookings, whistle(s), pen/pencils, assistant referee flags and a bag to put them all in. Many referee shops online offer bundles for all the essentials.
In regards to the tracksuit, you can buy one if you want to. If you're going to be doing junior football where you turn up already dressed it may be worth getting just a jacket/top. However, if you're going to progress onto open age leagues it's not a bad purchase to make. It looks better than turning up in casual clothes.