Lua is definitely the easiest to get into if you have never programmed before, but C/C++ is more useful knowledge to have since it's used on virtually every operating system and device and is a lot faster than scripting, and if you ever want to have a job in programming it's a very useful language to know.
If you end up going with C/C++ it would be best to start with programming something simple for PC before you try making 3DS homebrew, since the libraries there are a lot more mature making programming for the platform easier, and you could use a library like for example SDL to make game development easier. Once you are familiar with C/C++ programming for PC, it will be easier to get used to it for 3DS.
Otherwise Lua is not a bad choice if you are fine with just making simple 2D games and don't care about programming beyond 3DS homebrew.