"It is me!"
Translation:Jsem to já!
My take as a non Czech speaker:
“To” is the subject “jsem” is, self-evidently, the verb “já” in this case is the object
The direct object in a sentence would normally require the accusative case (mně), but not with the verb “to be”, which asks for the nominative.
“To jsem já” is a possible, and common, variant of this sentence. However, Czech does neither require nor have a strong preference for the SVO (subject-verb-object) word order familiar to speakers of English and many other languages. In this sentence the order has been switched to VSO (verb-subject-object). There are slight nuances indicated by word order (e.g., “— Which one of these is you? — To jsem já” vs. “—What / Who is that? — Jsem to já”, but I don't think this needs concern us at this point.