When growing up I played the violin and piano, worked at a bank, did weekly chores, had a lot of homework, participated in numerous church activities, and went to school ceremonies and presentations, among other activities. As I participated in all these things I found I had to buy certain clothing items, some of which I've only used a few times, however at the times when I needed those staple items, I sure was glad I had them handy and ready to go. Especially as an adult, I'm glad I know the staples I need for school, church, the workplace, and the social arena. Some fashions are simply classic and you just need them!
Some of the articles of clothing I found most useful and ones I found I wore all the time were my dress pants. Especially after I started working at the bank, I wore them everyday. The more I wore them, I bought a few more and acquired some hand-me-downs that still serve me well. Purchase a nice black pair (it's nice to have a straight pair and a boot-cut pair, but go for the straight pair first) and some good dress khakis. Especially if you work in a more professional environment, multiple pair of dress pants are a must. Good dress pants are a good and quick way to dress up an otherwise casual outfit.
Regardless of how often you wear it, have a classy black dress sitting in your closet. I really only wore mine for orchestra concerts, but it came in way handy and it looked amazing. You should also have a couple of versatile skirts around. I have denim, khaki, and black skirts that are handy for last-minute church meetings, quick violin or piano performances, and going to see your brother's school concert.
You need a few good pairs of jeans for social activities and you can also dress up jeans for dress-casual situations. You definitely need some good Saturday lounging type clothes. Have a pair or two of older jeans for yard work or cleaning or what have you. Also for Saturday wear have a few older t-shirts for just hanging out; I hate using a cute outfit on a day when I don't really do anything!
Really acquiring the classic fashion staples isn't too difficult because you'll find yourself buying them anyway, especially the social clothing needs (e.g. jeans etc.) You'll never regret it and always be thankful when you need something quick and it's just a closet away!