Booking HM/Travel agent ?'s

I'm wondering when I should start looking into travel agents to book my honeymoon. I have a general idea of where we want to go (australia / fiji). Also i've never used a travel agent before, so I was wondering how it worked.

Do you see a couple travel agents, to see what packages/prices each can get? Is there usually a charge for using TA's? How much is it?

THanks

Siobhan