What are some different ways to use これから?

Question

I know the basic usage of it, e.g. これから (thing the speaker wants to do), but there are some contexts where the usage doesn't seem to make any sense, like when someone just says it by itself.

Anyone have any ideas?

Answer 1

これから doesn't have the meaning "wants to do". It simply means "from now on" in most cases.

Example:

これからはじめます。-> I'm gonna start it from now on.
これからが[正念場]{しょうねんば}だ。 -> From now on, this is the moment of truth.

Reading Roy's comment, you can't understand what これからだ! means.

Of cource, これからだ! is not an exception in the rule I mentioned above, but I assume that the person who said これからだ! is in fighting scene.

Especially in fighting moment, some special sentences / shouts are often used.

Like:

かかって[来]{こ}い！ -> Bring it on! / Come on fight!
これからだ! -> I can fight more (even if you think I can't)! / I'm not defeated!
まだまだ! -> (Similar to `これからだ!` but more provocatively a bit.)

And they can be connected like:

まだまだこれからだ!かかって来い! -> I'm not defeated! Come on fight!

Answer 2

これからだ just means something like "This is where it starts" / "This is just the beginning".

In the context you give (someone gets up after being knocked down) a few expressions that could be used in English would be

That was just the warm-up!
You ready?
This is where it get's interesting!
I haven't even started!
We're not done yet!