Documentation on C# for FiveM is quite scarce compared to Lua, but id say try and learn the basics of C# and how you lay stuff out, then read up on This and try look through the forums or create posts if needed to get more information. You can find the native reference Here so you know what certain natives do, there is also sites for GTA:V modding which could also help, although the layout between the two when it comes to writing code for it differs a bit, you can still get the main functionality out of it.