Though it's not great for someone entirely new to C++, cppreference.com is my go-to reference while I'm developing.

IMO, the best way to learn is to work on a project.

nostr:nprofile1qqsvuxle4kfpvn0jy7lu4v5pxxfuvr45qgwnt06th0r05fx9vrg086gpzpmhxue69uhkummnw3ezuamfdejsz9rhwden5te0wfjkccte9ehx7um5wghxyecpzfmhxue69uhk7enxvd5xz6tw9ec82cs2mddwx, nostr:nprofile1qqsqxefne258ydmfgm2wfl02fsdqgs0d5wx29kweg9amxcqxew4t7kqpzamhxue69uhhyetvv9ujuurjd9kkzmpwdejhgtcppemhxue69uhkummn9ekx7mp0qy2hwumn8ghj7mn0wd68ytn00p68ytnyv4mz7tjlpwg, and I have started working on a C++ SDK for Nostr; you're welcome to contribute, if you want to practice your skills that way!