Hi there!
Welcome to the forums!
The first place I would point you in the direction of is the pinned posts in the FAQ sections. Have a read of them, esp about fishless cycling, tank maintainence and the like.
here and
here and
here.
Now onto your tank.
Neons are quite delicate fish. They are reccomended for tanks that have been set up and allowed to mature for at least several months. Coupled with this that your tank may not have cycled and you may have some poorly fish.
Give us some more info and we'll be able to help you some more.
What size is your tank (litres)?
What type of plec do you have?
Do you have a water testing kit? If so, what (exactly) are your readings?
Don't panic, I am sure we'll be able to get you on the right track soon!!