Translation:I found it!
Since there is an exclamation point, I would interpret this sentence to mean "I found it" in English. "Find" must be followed by an object or by a clause with "that". This applies specifically to "find". I mean, for example "Find out" COULD be seen in a sentence as "I found out", if you want to tell me about some information that we were both interested in.
I thought I'd share a play on words with you: A friend of mine in the States has a large picture on her wall of a matza board and the word מצאתי! across it. Unfortunately, I can't find a picture online like that to link here.
Referring, of course, to the Passover afikomen that the children look for after the Leil haSeder meal: a piece of matza that stands in for the piece of the Korban Pesach, the sacrifice it represents, that finishes the meal.
Because the את זה is not really needed. It can be used and is considered correct, but it seems to me (native English, immersed Hebrew) that את זה is an Anglicized expression in general. Some of the native Hebrew speakers here have expressed their frustration over it.
Besides, isn't it fun to know that there's another, even more authentic way of saying the same thing?
Εὕρηκα! Some languages drop object pronouns ("object drops" or "null objects") more than others in specific conditions. As a student of modern Greek, my understanding is that Greek can drop an object pronoun so long as the antecedent is clear, as could ancient Greek, famously with Εὕρηκα. Classical Hebrew of Mikra could drop the object pronoun, as is the case in Gen 3:6 ויאכל, "and he ate [it]." The antecedent, clear in context, is מפריו. Here's a link that discusses Japanese and Romance languages in which someone says that Brazilian Portuguese allows object drop to a greater degree than Portuguese in Portugal: https://forum.wordreference.com/threads/object-pronoun-it.3190254/