Assuming you are starting from scratch - start with the Lua programming language. It is very easy. Practise writing everything as a function, practise composing

Assuming you are starting from scratch - start with the Lua programming language. It is very easy. Practise writing everything as a function, practise composing everything from standalone, unit functions, practise writing recursive functions that use proper tail calls, practise writing co-routines. From there is will be a lot easier to pick up languages like OCaml, F# and Erlang - it will all be syntax, libraries and type systems.