There's a slight differente between "Ja ne!" (Later!) and "Ja nee". In the second you'd have to put more emphazis on the "eh" and make it sound longer... it's a very msculine way of denying something. Try to watch Naruto or Bleach, and when a male-macho character denies something, pay attention - he'll use "nee" instead of "nai".

For instance -
"That's not it!"
Female - "Sonnan ja nai!"
Male - "Sonnan ja nee-!"

"I won't eat it!"
Female - "Tabenai!"
Male - "Tabenee-!"

See? The "nai" becomes a nee-.

Which is different from "ne". "Ne" is used mostly by females and, when making a rethorical question/confirming something, by males. It doesn't mean much, and of all the particles, it's the safest to use.

Also, careful with your "I" forms.
"Watashi" is the safest, since is neutral and formal.
"Atashi" is only for women but it's informal and not to be used with just anyone.
"Boku" is for men and is polite, therefore safe to use.
"Ore" is for men and also a big No. Too informal and rude if you use it with strangers.

There are more, but those are the important ones