I don't know where to go next

[Si600]

I have a little riff that both the guitarist and I think has potential, it'll fit as an intro, bridge and outro but I need some thing for the verse and chorus. It's F G C A#.

How do I know which key it's in, I think F as that's the first note.

Following from that, can I use anything in the rest of F Maj as the rest of the piece or should I just say stuff it and noodle around until I find something else that sounds good and tack it on and hope?

It'll be in about four different keys if I try that, does it matter?

[Si600]

For want of any other theory knowledge I've called it in F and gone:

F G C A# as previously intro etc

A F C verse or chorus

G D E C verse or chorus

It sounds good to me!

[Newfoundfreedom]

3 minutes ago, Si600 said:

 

It sounds good to me!

That's the only theory you need. 

[Higgie]

No way of telling what key it's in until we know what the chords are, not just the root note. But it looks like it could very well be in F, if you rename the A# as Bb.