Fxaix and contribution question

Hi everyone! I (19F) just opened a Roth IRA, and I'm trying to figure out a couple of things. I tried google but had limited luck so l thought I'd try here! :) I've done some research and am almost fully set to go with the FXAIX fund (unless anyone has a better one), but now I'm confused about how to invest in it. 1. Do l need to spend the ≈ $200 on a full share, or can I pay incrementally. I'm only a freshman in college, so I have a very limited income. I'm wondering if it's better for me to invest smaller amounts monthly rather than saving up for a lump sum each time which would take a long time. 2. When I went to transfer money into the account, it gave me the option to contribute for 2024 or 2025. I'm confused because already 2025. Is this an error? If not, which year should I contribute to first? I'd appreciate any advice! Thank you!