For A1, I'd recommend mastering the pronunciation first, because nobody wants to have to relearn all their vocabulary after 10000 words.
As far as grammar goes, ser vs estar. It's tricky at first and it just keeps getting tricker as you advance.
And, of course, all the different endings on all the different verbs in present tense. It's a slog, but you'll be happy you did later.
If you are working on your own, I'd also suggest to you looking at the subjunctive present tense, too. The subjunctive can throw English speakers when they meet it later down the road, so if you get the idea (just the idea!) and learn to recognize the most common verbs in their subjunctive forms now, then you will make life easier for yourself at B1 level. You don't have to be able to use them, or have them mastered in any way....just wrap your head around the idea!
I think that basic vocabulary and common expressions are the most important things to work on and practice in A1 level.



