So I was looking around AO, thinking of what to do when I thought, geeze, I should keep the public tutor going. I hope this will catch on soon though.

Now, I don't claim to be a guru but I hope what I say is insightful enough to help at least one person. I'll be talking about text today, in some detail and to some length. Nothing intense though.

What is text?
Text my friend, is the writing within the art.

Do text and font mean the same thing?
Nope. A font is the type of letters you use to make up your text.

What's a font then?
Ever heard of Time New Roman? That's a font, same as Arial and everything else in that drop-down menu.

How many fonts are there?
I have no idea but I can tell you there are many. And to answer the next questions, you download them from the net.

Those are pretty much the basics I think. Let's talk bidness now.
I want to start by telling you that text in the art isn't always necessary; some art looks better without text anyway. There are many important things to know about text even before you place it in the sig.

One should add text into the sig in the sig and not on the sig; don't just add some text after all the work is done. However, this rule, like most, can be ignored if the text you add makes the sig more beautiful. I find that working the text into the sig looks nicer but that sometimes gets in the way of blending and other elements.
Let's say you were making a very smudge-intensive tag, this is where I find adding the text last seems reasonable.

There are many things I can tell you to follow but you may ignore them and it's all because of the Rule of Thirds. Personally, I have never used that particular rule myself but I'm familiar with it. You should be also.
I can tell you that placing your text near the focal is a good thing. The focal point is basically the best part of the sig or where your eyes are drawn to most. Get your text around there but not intruding upon it, have it add to the focal and make it better with its wit.
Text hidden under elements of the sig also add a certain flare. The words can be obscures, running off the screen, barely noticeable, etc. As designers we make art so others can see it but we have certain liberties I think, and one of them is the freedom of text.
The greatest tip I can give you is this: DO NOT PUT TEXT WAY OUT IN THE CORNER NEEDLESSLY. It looks ugly and it draws the eyes away from the focal. This ties into my second point.

It matters greatly what color you choose for the text. Often when I see text offenders, they have colored their text in brashly. It clashes horribly with the sig and almost destroys the whole effort. What I'm saying is having a nice, mysterious sig full of rolling purple smudges set onto a off-black background and smack dab in the middle - neon green text. Why trouble yourself with making a ponderous background then ruining it with some so obvious? Atmosphere is ruined.
Colors that melt into the sig are always a good choice. I didn't say this yet and it may contradict my last paragraph but adding off-set colors into your text works too. Off-set meaning something just a little bit brighter or darker than what's in your sig. And that brings me to my next tip: use the colors from your sig/render! If you're using a render and it has a menagerie of lovely colors in it take full advantage of them.

Moving on, the size of your text also matter but to a slightly lower degree that the above. I don't often see people experiment with size it's pretty much under 20. Some fonts, especially cursive ones, demand that you like, super-size it so its visible so that's exempt I guess.
One trick I love seeing is bold fonts made large and little decorative fonts, and all others pretty much, downsized and neatly above the large font layer. Size is something hard to preach seeing as how it's pretty self-explanatory. So.. I'm going to wrap this part up now.

Text are words and words have meaning. Meaning put something imaginative there, that relates to your sig. Please, don't think I mean that if you use Naruto you have to put something obvious like 'I'm Number One.'
Still using Naruto as my example character, imagine the sig is of him; there are cool effects going on in the sig and some pretty decent depth, blending is killer and the dominate color is... green. Do you think putting 'Konoha Magic' in the sig sound better than 'I'm Number One?'
I hope you said yes.
Cleverness often evades us, mostly when its needed most but it's not truly hard thinking of something witty and fitting to put in the sig. Never too long but always true to the sig your words will describe whatever you want. It could be meaningless or meaningful, it could even just be your user name.
What I'm getting at here is try and be creative or if that muse just isn't coming down off her high horse, whatev, Leave it blank, put in you name or something, wait for the perfect sentence. Cope.

I've probably gone against every bit of advice I've just given you at least twice but I assure, above there are some genuine tips I would have killed for as a beginning designer.

And here's where I wrap things up homies! I'm glad if I could help and if there are errors, whatever. Unless the error is so grave it makes reading this impossible then please, tell me.