There are many people who actually get into HR after graduating in a totally different stream. To some people it just happens while for others it is a deliberate move.

If it just happened to you, you can say that I was on a look out for a job when I got the first opportunity in HR. I took it up and slowly realized that I have started liking it. I brushed up my skills by keeping myself updated about the new developments in the field and acquiring a professional degree (If you have got one.)

If it was a deliberate move, you can discuss your reasons for moving into this role.