Hello Simm, and welcome to English Forums. I hope you will register as a member (it's free!) and join our discussions frequently.
The indefinite article (a, an) is determined by the first
sound of the word, not the letter, so we have--
A unique experience ( /
ju:nik/ )
An umbrella ( /
umrela/
A haircut ( /
heirkut/ )
An heir ( /
eir/ )
etc.
I hope that helps.