This is probably a stupid question but I've been trying to resolve it for a while now and due to low knowledge of music theory, I can't. I don't really know what key I'm in.
One way to do this is list out all the tones of the chords and then find a key signature they all belong to:
D# F# A C#.
So follow the circle of fifths for sharp keys until you get a match...
1#: G/Em - F# 2#: D/Bm - F# C# 3#: A/F#m - F# C# G# 4#: E/C#m - F# C# G# D# ...
You should see that your set of notes matches
That's the 'mechanical' part of the issue, but now we get to the actual important part: there isn't a single answer to this question. Or, you might say it isn't answerable.
E we could continue the harmony as
ii viio6 I or
ii viio6 V7.
C#m we could continue the harmony as
iv iio6 i6 or
iv6 iio6/4 V7.
There are other possibilities.
You probably expect the answer to simply be 'it resolves to
E major, because that 'resolves' the diminished triad treating it as the leading tone triad
viio. But it's really wrong to think that is the one answer.
A nice solution would be:
Starting in A :
A - f# - d#dim - A/E - E7 - A -> I -vi - (vii°7) - I46 - V - I
d#dim would be a secondary vii°7 to A/E (I46) with the final E - A = V - I