Hi! like others have said, i would recommend you get a small one. First, they are cheaper compared to fully grown ones and their eating habbits may vary. But this is entirely up to you. When you do get one you can feed it crickets, baby goldfish, uncooked prawns and several others. My opinion its always best to feed it live food. Another thing to note is when feed a fully grown arowana live baby goldfish you would want to chuck one at a time. If you chuck in about lets say 5 fish in the tank at once chances are there would be survivors that would swim near the bottom of the tank. Having such thing happened the fish would always look downwards to view its prey, and knowing that it is a top feeder it is not very good on its eyes. Continuous starring to the bottom may cause eye injury to the fish, as i said it happens over time. My mates fish needed eye surgery because one of the arowana's eye flipped over. well anyways hope this helps..
Cheers Dan