In the Tractatus, a propositional sign is not the sentence type, as Allegranza suggests. In fact, it need not be a sentence token, either: It is whatever particular fact that makes a proposition available to the senses. So, it might be an arrangement of audible sounds, visible ink on a piece of paper, etc. (Wittgenstein stretches the notion of picture, or proposition, very far, so accordingly the notion of propositional sign is very wide.) Of course, the propositional sign is not an event, e.g. your hearing the sounds, or your seeing the ink, but the particular configuration of elements that flow through the air (in the case of sound) or that is marked on the paper (in the case of ink).
So, "p" and "p" are two different propositional signs.
Likewise, Wittgenstein's conception of the proposition is not the Russellian one. What a proposition shows, to anyone who understands it, is the possible fact that would make it true. Whether or not this possible fact is a fact 'of the world', i.e. exists, depends on whether or not the proposition is true. The proposition says that this fact actually exists.
So, there is no question of how the proposition can 'exist' if it is false, since the proposition is independent of the fact that makes it true.
The proposition itself is also a fact. Propositions are logical pictures or models of reality which we gather from the propositional sign by analyzing its internal structure, in order to arrive at a logical form, a structure of constituent signs. The logical form projects a situation independently of form of representation (i.e. the form that the propositional sign/picture takes, including whether it be an arrangement of colour, sound, shapes, etc.), because we look for that very same logical form in the possible fact (in the world).
So the proposition is not really a different fact from the propositional sign, but is just the "pictorial sign in its projective relation to the world". It is, as it were, the propositional sign considered as a model, rather than just a bunch of sounds or marks.