Interjection is treated as the last part of speech in traditional grammar, whereas in the modern grammar it is not included as the part of speech.
Interjections are those words which sometimes don’t have any literal meaning (like; hurrah, wow) and they are uttered spontaneously to express sudden emotions of; grief, surprise, wonder and pleasure etc. It is something like a reflex action that we are unintentionally used to utter them in sudden happenings.
There are certain situations in which we can’t help uttering these words, like when we come across an unexpectedly wonderful situation we spontaneously utter [Wow!]. Similarly at grief or sorrows we say [oh!], [ah!] or [oh my God!] etc and [shit!] when something goes wrong. All these words are interjections.
oh!, ah!, bravo!, wow!, hurrah!, shit!, damn it!, hush!, what! etc.
Sometimes the whole sentence express emotion like “Oh my God!”. Some interjections have literal meanings too and they are not called as interjection unless used with the sign of exclamation, for example look at the following words;
In the case of example  you see there is no sign of exclamation and therefore it is not an interjection because it doesn’t seem to express any emotions and should be read in level tone. Whereas in the case of example  the sign of exclamation makes this expression unique and we get to know that it is not simple matter and we also read it in rising tone as if we express over excitement.
Sign of exclamation (!)
After interjection sign of exclamation (!) is always used. In fact sign of exclamation itself indicates an expression of emotion. So it is necessary that it should be used after interjection. Even in simple sentences when the sign of exclamation is used it turns to be an emotive expression, for instance look at the following example.
Now in the second sentence you might have felt a feeling of joy, whereas the first sentence is not like that. It is just a simple matter. You will even read it likewise to show a kind of excitement . Excitement is always expressed in rising tone.