It’s like starting from scratch with every hobby: if you run out and buy the best of the best for every occasion without dipping a toe in, first, you’re going to be deep in the red for a while, if not until you run out of steam.
You can get a T shirt for $5-15 easily, but aside from bigger sizes/mens and womens, there’s not much deliberate variation you’ll see in mass produced stuff, so if you want clothes to really fit you, it helps to learn how to either alter them or make them yourself.
There are plenty of kits you can buy that have sewing patterns ready made. Here is an example. You follow the instructions and the patterns have some leeway in terms of customization. Play around with those and you start to figure out how the changes you make affect the clothing and how it fits.