Translation:This is my letter.
Grammatically, you can use it after practically any noun or verb where you could otherwise use "saya" or "aku" You can also use it in front of /some/ verbs, but it's easy to get that wrong.
You can't use it on it's own eg. at the start of a sentence.
Keep in mind that it's much more casual than "saya" so don't use it in formal situations.
"Ini surat saya" is sufficiently formal for most situations.. "Ini adalah surat saya" is grammatically correct, but I'm not sure what circusmtances, if any, require that level of formallity.
"Ini ada surat saya" is grammatically incorrect and I think that "ini surat punya saya" is too. For the latter, you might have been going for "Surat ini punya saya" = "This letter is my property" = "This letter is mine"