You should start by working through the installation guide, naturally. After this it is recommended that you work through one or both of the tutorials, this should give you a feel for basic TextTest usage.

It is generally useful also to read through the material in the Basic Setup section, this will tell you what you need to know in order to get TextTest basically up and running on your application.

The User Interfaces guide is there as a complete guide to the GUIs, which are more configurable than you might imagine. Read it if understanding the GUI isn't going too well...

The rest is mostly summaries of additional features of TextTest. It can be worth reading just the introduction to each of these to see if the features seems like something that would be useful to you. Reading further is probably only useful if they are.

Besides being a standalone tool, TextTest is also a framework written in the Python language. If you are a Python programmer (or have access to one) it is probably worth browsing the framework documentation.