You can start by learning basics of Lua Language. Just search on Youtube, tons of tuts there. Also you can find lots of resources here: Big List of Lua Resources!
and then you can start learning Defold from tuts:
https://www.youtube.com/watch?v=Ri2KStY-mfE&list=PLseKVnAXs_iVaksj-sjkz6R1D9Fpv1EaT
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