Questions about starting Roth IRA

(24 M) I’m gonna start my Roth IRA because this year I’ll be Maxing my Roth TSP and I feel I should have a Roth IRA as well here are my questions

1- what app/ website is recommended I’ve been hearing about mostly 2 (Vanguard and Fidelity)

2- If I start contributing today would I be going off of the 2024 limit or the 2025 limit because I read it restarts in April but I’m not sure if that’s only if you starting your contributions the previous year