Two Airlines are operation to Lakshadweep island from Kochi. Indian Airlines and Kingfisher. Only Agatti island has air strip ( air port) . So check the availabity of tickets from kochi to Agatti island in any of the two websites or travel sites and book it.
There are helicopter services availbale from Agatti to Kavaratti and Bitra island. But make sure that the helicopter services are available to you when u reach the Agatti island. You have to enquire at Kochi office of Lakshadweep administration . To be frank I do not have good idea about availability of helicopter tickets. The tarriffs regarding tickets are availbale from the official website of Lakshadweep administration.
During fair weather season -i.e from Septer end- you will get passengser vessels to other islands from Agatti.