Summarize the following:
The thread must be smooth and firm. The mercerized thread will provide both sheen and ease of movement along the tatting shuttle. This is a fairly standard tatting thread that works well. A common brand is Coats Mercer-Cotton. Crochet cottons are numbered and the higher the number rating, the finer the thread is. Knowing the rating is important, as different projects require different thread weightings and different levels of ability. For example, very fine thread should only be worked by the very skilled and advanced tatting enthusiast, whereas thicker threads will be far easier for beginners. As such:  100 is very fine thread 10 or 20 are very thick threads 40-80 are considered medium threads and tend to have a good range of colors.

Summary:
Choose thread that is tightly twisted and highly mercerized. Consider using crochet cotton. Know the thickness ratings of the cotton.