Usually a lot of people will say decide on ur budget first... But I find it so hard to! Anyway, I would say, go shop ard for bridal studios now, even though u are only planning for ROM. Cos bridal studio packages will usually include trial makeup which u can use during ur ROM, some may even throw in a ROM gown. Then u shd also consider on e venue.. That will be a bigger headache, but usually e prices, size and preferences kind of narrows it down for u.. Once u have these 2big item settled, u go find ur JP available on that day, timing. Then, photographers, ROM cert etc comes in..
Hope that helps!