Two different ways of checking slab deflections in Tekla Structural Designer are available. Either deemed-to-satisfy checks, or a rigorous theoretical deflection estimation. These are introduced in the topic Slab deflection methods.
If you decide to adopt the latter approach the video links at the bottom of this page are a good start point, and the Rigorous slab deflection workflow topic also provides an overview of the steps required.
Many topics are then discussed in more detail in the Slab deflection handbook, which also provides Tutorial worked examples for you to download and work through.