i mastered every katakana and hiragana, what do i begin with know?
For example, 一 =1, 二= 2, 三 = 3 ,人 = people etc.
But if you're still beginner, you'd better to learn simple grammer or some greatings, like Kon nichi wa = hello , Arigatou = Thank you etc.
It depends on the purpose.
Hello! congratulation on mastered Hiragana and Katakana! SUGOI!
Now you can read Hiragana, so, I recommend you to buy some Japanese textbook for beginners.
I think "GENKI" is widely used by whom speak English.
With textbook, you can learn fundamentals of Japanese like greeting phrases, basic grammer, and so on.
But I think it is difficult and not fun to study only yourself, so you should find good teacher or language exchange friend!:)