我明白我该怎么做了用英文怎么说

I understand how to do . 我明白怎样做了（我知道该怎样做了。

I known that whatever i must do !我知道不管怎样我都要一定这样做的。
must 是一定的意思。

Now I know what I should do.

或者说：
Now I know what to do.

说法有很多种的，口语中倾向用I've got what I must do

I've got what I must do.