For starters, you have used the notes A♯, B, C♯, D and F♯. The problem is that they won't belong to a diatonic major scale. The reason for that is that any key with an A♯ in it, will already have a D♯. And your tune has D♮.
We could look at those sharps as flats: B♭, C♭... - no, that's not going to help!
So, we look at minors, and that A♯ to B is a clue.It features in both the harmonic and melodic (rising) minors, where those two are the leading and tonic notes. Of B minor. The B minor scale goes B, C♯, D, E, F♯, G, A♯, B - or B, C♯, D, E, F♯, G♯, A♯, B for the melodic. So, the key signature will be simply two sharps, as in B minor, and each subsequent A note will need a sharp before it. And B will most likely feel like 'home'. I hope there will be more of the tune than just five notes!
EDIT: just a thought, but if the A# was the anacrucis, putting more emphasis on the B at the start of the next bar, that would make it sound like it is in B minor. What do you think?