Could be any number of things.
image> image mode
only just started using the forums in the last couple of days
welcome! :)
(dave m - I'm at work so I don't have all the answers posed above. I'll know more this evening.)
Also, there are basically two types of Text: Paragraph Text, where you first drag a Text Box, and what I'll call "Open Text," 'cause I'm having a brain cramp and cannot come up with the proper syntax. There are some differences in the behavior of the two. The manual (do they still ship those, or is F1 the only recourse nowadays?), should make the distinctions clear.
Also, when Text does not behave, as you think it should, look at your Layers Palette. Since about version 5, PS creates a Text Layer, when you start using the Text Tool. Make sure that you are in the correct Layer. If it doesn't seem to be working still, Dbl-click the icon with the T in the correct Layer. You should see your Text selected/highlighted. With the Text Tool, you can edit, or add to from there. This gets you on the correct Layer. If you first just click with the Text Tool, you are most likely creating a new Text Layer, which is not what you want, if you intend to edit, or add to existing Text. Always keep the Layers Palette open and in view. When things don't work, check to see just which Layer is active. That is NOT just for Text, but for almost everything in PS.
Good luck, and let us know how it goes. I also like the "Classroom in a Book" series from Adobe Press. They step you though many aspects of PS in little "projects." They are fun, expose you to things that you might not find for a bit, and are highly informative.
Hunt
As a total aside - and for whoever is reading this - I am amazed at the generosity of people in these forums. I have only been doing this for a few days but I am stunned (I can't think of another word) at how generous people are with information, and how friendly the tone of this forum is. (I believe you're not suppoed to pontificate on these sites but I just had to say that! I promise I won't do it again.)
So, last night first I looked at the Image Size, learned more about the image, set the font size accordingly and then simply clicked (without opening a box), started typing and voila! there was my text.
And yes, working with Layers helped tremendously to 'see' what was on the image.
(Hunt, I'll try the 'Classroom in a Book' series.)
Thanks!