New budget build: any advice welcome

Hey y'all!
I just got a hardtail frame off of Facebook, and I'm doing a build on it, trying to stay under $400 dollars if possible (cuz that's all I have) I'll for sure be coming here for specific advice regularly, but if y'all have any advice on something you wish you'd done on your first build pls do let me know! Also if you have any leads on really good deals on parts, I'd love to be informed of those too.
Thanks
Gunnar