First, by online it means you run the agency transactions(Bookings, Enquiries etc) via your website- so you must start by building a website with this end in Mind. Secondly, enlist properties be they Hotels, Tour Packages, Flights or even car hire. Since you are just starting, you might want to build contacts with Hotels for instance where you will direct clients upon booking before you can build xml based systems for realtime booking through e.g through a vendor. Thirdlly, it is a very competitive field out there especially for online travel agents - brace your self for a serious online marketing campaign given that your business is located in the online space. Fourthly, remember, Most importantly, in the online world, consistency pays. With the fierce competition and hard work comes good rewards for those who come out on top.
Wish you success.