My understanding is that you need to register to be a delegate with your county GOP.
Mine was Spotsylvania. It was as simple as selecting what you wanted to be a delegate for. I picked District and State convention.
Then, you will be notified and invited to attend your District convention as a delegate from your county.
UNFORTUNATELY, I'm fairly certain that the STATE-WIDE deadline for registering to be a delegate was Apr. 13th. At least it was for my county, which is both in the 1st and 7th.
Check your county's GOP website!