finallyflying:
Do you have the required IATA and ICAO Codes for your organisation?
If you do then there are a number of options for SITA and as Grasscarp points out the Place to start is
Web Type B | SITA.aero you can then use the SITA Gateway for your AFTN Message (providing volumes are small).
If you don't have an IATA Code for your Organisation and believe that you don't need one, then maybe
e-SITA from Wrengler would be adequate, This would still give you access to AFTN via the SITA Gateway.
You posed what appeared a simple question but with all the possible access solutions from SITA I think you need to think about the numbers of messages you will be handling a month and therefore the costs that are likely. At the end of the day part of SITA you would deal with is now a commercial organisation.
I am not sure it helps, but back in the Archive is a thread which includes a Post I made back in May 2009:
AFTN or SITA address structure