It's pretty straitforward.
If you hit the plain "T" tool click it anywhere on the canvas, and start typing, you will see type, unless;
1. Your type is the same color as the background
2. Your font is corrupt, in which case, choose another font.
If you are using the T tools that are either enclosed in a box, or that have a line underneath them, they do not apply unless you are filling an object with type, or tying type to a line/curve.
Like I said, you have to click the T tool and then click anywhere on a layer that isn't locked.