There are two simple ways to access NDC content if you are not yet an IATA agency: GoLite and TIDS
Both solutions can be useful depending on what you need in your travel agency. Example: if you would like to access Singapore Airlines NDC content only, you can request a TIDS number, and you will be able to visualize the content for free. If you would like to get all the content available in NDC, and issue and manage your own tickets, GoLite would be the most relevant solution.
To help you decide what is the best solution for your business, here is a summary of the key services provided:
How to register with IATA