Lila I've answered this question twice before and I firmly believe that if you don't read Percy Jackson first, you won't like the Heroes of Olympus as much. Hope this helps!
Ranee Babu I think it would be best if you read the other series as well, because there will be somethings you won't understand.
Katie You don't HAVE to in order to understand and enjoy the book (all new main characters, all new plot), but I think anyone who did read the original Percy Jackson series is going to enjoy the book a lot more. Also, if you ever plan on reading the original series read it first to avoid spoilers.
Kenzi Staley you can read it before but you well get spoiled on the percy jackson series so it is better to read PJO before HOO