I'm working on a big project and I'm wondering how to make an item have a description like potions, I haven't tried any code because I have no idea what to try. Please help.

Have you searched for some tutorials yourself? That is the first thing that you have to do?

@SideOnly(Side.CLIENT)
public void addInformation(ItemStack stack, EntityPlayer player, List list, boolean par4)
{
list.add("your description");
}

Thank you so much. 

@SideOnly(Side.CLIENT)
public void addInformation(ItemStack stack, EntityPlayer player, List list, boolean par4)
{
list.add("your description");
}

 

You don't have to just give people code, they need to search/research. Like he can look at the potion class and search there.

I did look at the potion files and didn't find anything...

I think the description of potions is added in snapshot 1.6 MC version...

I think the description of potions is added in snapshot 1.6 MC version...

 

There's already some description to the potions before 1.6. You need to look at the actual ItemPotion class, not at the Potion classes.