[KyleGoetz]

Tsuwabuki, to quote Winston Churchill,

"People telling me I can't end sentences with prepositions is something up with which I will not put."

The "stylistic" prohibition on ending sentences with prepositions was made by silly grammar teachers importing rules from Latin into English in the early 1900s, if I've got the era correct.

Nonetheless, it is true that ending sentences with prepositions separates formal writing from informal in the minds of many people who matter. Thus, it is useful to obey the rule if you want to impress with your writing in formal settings despite the fact that it's a stupid rule that only idiots adhere to all the time.

And to get back on topic, I personally think you can't just focus on one thing (for too long) at the exclusion of other aspects of language learning.

But if you must, I'd say that you need to focus on a base level of maybe 100 nouns and verbs, and then you move on to some basic grammar (conjugating, changing mood, and a few auxiliary verbs and such). Then from there you need to learn more words. After a while, you'd better have started learning kanji, becase you're going to hit a point where you will learn words more efficiently if you know 1200+ kanji for this reason: Eventually, learning new Japanese words based on kanji will explode your vocabulary like learning dis-, un-, -phobia, -philia, circum-, -scope, -phone, etc. in English explodes your vocabulary.