that is really a loaded question. do you have a host, MIDI interface, cables, etc.? how much money do you want to spend? what kinds of features do you want?
if you're asking here in the Arturia forums, i guess it would be right for me to at least recommend their cheapest solution--analog experience: the player.
it features a compact MIDI keyboard controller and light version of the analog factory software.
without knowing your amount of knowledge on the subject, i would point you to computer music magazine
which often sells a good getting started guide. kvr audio
has great forums filled with information on music creation. harmony central
and many other websites can help you with reviews for finding a suitable MIDI keyboard. the hitsquad musician network
is a great resource for finding freeware, open source, and shareware music making software. hopefully something listed there will point you more in the right direction.