I started with Target and Amazon. I was working at a software company then and had disposable cash so I kind of just bought whatever I thought would look nice and then tried them all on for a day. I still have 2-3 dresses I don't wear but I look back on it like, "Oh I remember that phase."
My suggestion for tomboy look is, as you already started, getting those tight jeans. As for shirts, I have two recommendations. Really big t-shirts draped over you look like boyfriend tees, and that, at least to me, looks very feminine (even if you're not interested in men whatsoever, like if you're hella gay like me, the boyfriend tees to me looks feminine). Another big one is just making sure you get a woman's shirt instead of unisex. I have a bunch of women's shirts that really show off my figure and make me look a lot more feminine.
Quick tips:
Don't do short skirts or short shorts right away. Can lead to some awkward situations.
Always wear panties/underwear, and if you're wearing something more revealing (skirt/dress), wear a slip or leggings.
Leggings aren't pants. I'm sorry but no.
If you're bra shopping, get something really cheap that fits. As your breasts grow you'll go through bras like crazy. I suggest getting sized first so you can get your number, and then get a sports bra that will comfortably fit you from your number, A to B. Then just get more sports bras over the years of development. The route I took was more about the cute bras, and they were nice, but I can't fit into any of them anymore.
Also, just do whatever makes you feel comfortable. Like seriously, you're in charge of your body. Take our suggestions or leave them, whatever you do in the end, just make sure you're doing what you want 🙂