Placed in the begining of a sentence, it will always be "à". "a" without an accent is a verb.
A good way to know for sure (I learned that in school!!) : If you can replace the word "a" by "avoir", there should be no accent, as it indicates it is the verb. If you can't, you should put the accent.
-Je vais à l'école. "avoir" l'école is wrong, so an accent should be placed on the "a".
-Elle a l'air heureuse. "avoir l'air heureuse" is correct. There should be no accent there.
I hope it makes sense!
edit: oups! I didn't understand your question right! Sorry about that! As a capital letter, both are correct as cropje jnr mentionned.