Good pronunciation should be one of the first things that you learn in English. You can live without advanced vocabulary — you can use simple words to say what you want to say. You can live without advanced grammar — you can use simple grammar structures instead. But there is no such thing as “simple pronunciation”. If you don’t have good pronunciation, you have bad pronunciation.

And the consequences of bad pronunciation are tragic. Even if you use correct grammar, people may simply not understand what you want to say.

For example, if you pronounce sleep like this, and not like that, or if you pronounce ghost like this instead of this, native speakers will have serious problems understanding you! In my opinion, you should know how to say English sounds like the ee in sleep or the o in ghost, before you even learn words like sleep and ghost.

Here is an anecdote about this. After coming back from a vacation in the United States, a friend of mine said:

Whenever I spoke to a person in America, they kept asking me What? What? I would repeat my sentence again and again. Finally they would say Ah-ha! and then repeat my sentence, using exactly my words! It was very humiliating. I knew my words and grammar were good, but nobody would understand me, just because of my pronunciation. I am very motivated to learn English now.