Imo, the best way to learn anything is by practicle approach. Same goes for arduino.
Gather some arduino UNO, Nano boards.(You don't need to get original ones from their website. Just buy some clones[Remember: to read customer reviews.]). Learn the very basics on C language(Variables, Functions, Data types, Conditional handling, Loops are enough) .
Then choose a small project(Like blinking an LED) . Mark down a rough sketch(Inputs, Outputs, Values, Shortened notes about sensors). Afterall write the program using Arduino - Programming Reference. BOOYAAH! Test it. If problem occurs, Google it. Always think something new, Be creative, See other projects, Analyse the code. Happy building 😊.