PART 1 of 2
It’s very motivating to start using the language right away, but yes, you’ll want to learn the basic grammar (i.e., how to put words into a sentence) ASAP. Otherwise, every time you speak, you’ll be practicing bad French. The fact is, over a quarter of English words are from French, and an additional number of English words (more than 50% total) are directly from Latin (the mother language of French). This is why learning vocabulary should not be the focus for a native English speaker learning French.
Of course, there are a few hundred very common words that are different, and absolutely need to be learned. Guess what — most of these super common words are actually structural — i.e. grammar. You cannot just learn “le” means “the” — you need to know exactly when to use which article (gender, number, definite, indefinite, partitive…). We cannot just learn that “to be” is “être” that will never work. It’s not 100% necessary to know the terminology (although it’s easy enough to pick up, and can save lots of time in the long run), but you do need to practice using grammar to express your ideas (out loud, in real time), rather than just floundering around during a conversation. This will also help with your listening.
What makes English and French different is principally the grammar, as well as the pronunciation. Almost all of the content words in the last sentence are the same in French:
Anglais, français, différents, principalement, grammaire, prononciation.