Alright, here is the basic way you have to find one. As soon as you get the bounty don't expect one to just fall out of a tree and hit you in the head. It takes a moderator to post you found one in other words you have to go on adventures and happen upon one. Once you find one the moderator will post the type of devil fruit it is...Logia, Zoan, or Paramecia type. From their you can give mostly any ability you want as long as its not a pre-created devil fruit and it fits into the devil fruit category you were assigned. Even so just because you find one doesen't mean you have to eat it!
Along with that note a few other things, you can sell it keep it for those "just in case situations" or give it to a friend. Even if your friend is say $5,000 bounty....doesen't mean he can't eat a devil fruit you give him! He just can't find one on his own. Be creative with your devil fruits...but no a few more facts.

-If you eat two your body will basically explode....I DO NOT SUGGEST THIS (I also don't need any of you trying to feed someone two pieces while their sleeping to kill them -_-)

-There is only one of each kind.......so don't try to create one already made.

-It doesen't matter if you eat the whole thing...the first bite is all that matters.