First of all, congratulations! I know backpacking can be intimidating (and expensive) at first, but it can be immensely rewarding in many ways, so its great that you have decided to take the plunge.

For me, I ditched my whole rain "outfit" pretty early on. I had armed myself with a rain jacket and rain pants. But if I tried to walk even a mile in that stuff I would sweat so much that any dryness was negated. I instead used an umbrella and a rain skirt, which I found gave me the same amount of rain protection (which, no matter what you wear, you are probably going to get wet if it rains enough) without any of the uncomfortable heat and sweating. I found this set up to be superior in almost every way. Here is a picture of my (silly by highly functional) outfit: https://imgur.com/a/86NkaW9. I would say the umbrella was more valuable than the skirt. The umbrella was also GREAT in Pennsylvania farm country in July, where it served as a shade-provider for days on end.

In terms of invaluables: Water filter has to be number one. I love the Sawyer filter (can get at any walmart). My luxury item that I would recommend to anyone starting out is an inflatable pillow. It is probably not necessary but I really enjoy it and it is small and light weight. Obviously tent, sleeping bag, and sleeping mat are must-haves as well.