Using Skills against PC's

[Queenie]

I have been told there is rule that says you can't use your skills on your fellow PC's, only NPC's. Could someone direct me to where it says that? Thank you!

 

[billd91]

I have been told there is rule that says you can't use your skills on your fellow PC's, only NPC's. Could someone direct me to where it says that? Thank you!

That depends on which skills you mean. Hide, Spot, Heal, and most other skills that don't entail forcing another PC to act in a particular way work normally on PCs as well as NPCs. The interpersonal skills have only limited effect against PCs because the player is always in control of his or her character (barring magical compulsions and charms and so on). See page 15 of the DMG under Handling PC Actions for some guidelines.
In general, any interpersonal skill used to get someone to act or believe a particular way isn't for use on another PC. That's generally Diplomacy and, in some instances, Bluff (though Bluff also allows a PC to keep a straight face when another PC tries to determine if he's giving off clues that he's lying).
Most other skills, though, work just fine but keep in mind that there's a difference between in-character use of these skills and inter-player use of the same skills. It's up to the player to convince the others about the wisdom of his plan by using his own personal skills of diplomacy and debate and not his character's roll.
Now, if characters debated, I'd have them roll Diplomacy to see who was more eloquent. I might tell the other players, "Wow. He makes a strong argument. Most normal observers would be solidly persuaded by it." and leave it up to the player involved to decide whether his PC would go agree or not.