One of the easiest is dice. You can easily manage the risks by lowing the win amount while learning it. Also, it's quite transparent that makes dice a good choice as an option to begin gamble with.
I agree with you on that, I think OP should ask what's the easiest online games rather than the safest because there is no safe in gambling, regardless of the amount we are gambling, we are risking money on that, and we could possibly lose more if we loss control along the way.