Help with Greek letters!

Please someone tell me what is the difference between these letters because they pronounce the same, but is there any rule on when to use one instead of the other?

Ο ο and Ω ω
σ and ς
Η η and Ι ι

Thanks

well that's a good question) from what i saw( being a self didact in what concerns greek language) i saw that "σ" is written like this at the start or at the middle of the word, and like this "ς" at the end...
But i told u, this is a thing that i noticed We must wait for a better reply) a smarter one:P
bye

what kreatza said is right, 'σ' is the first letter of the word or it's in it, and 'ς' is the last letter of the word (if this is a 's', of course)
you pronounce them the same way

the other letters I don't know, I'm not Greek and only speak 5 words of it (I'm trying to learn it)

Kreatza was right with the σ and ς, the rest im not too sure, lets hope we get a reply soon.