You need to focus on legibility and placement. I would HIGHLY suggest taking a typography course (or find a professional one online in a... "less than legal" way). You'll benefit a lot from it if you're doing things like this.
When you pick fonts, decide how it'll incorporate into the picture. Chances are you wouldn't put a sans-serif font for a luncheon invitation, and you wouldn't use serif font for a child's toy store. You might also see sans-serif fonts for very elegant companies, too, but you'll realize they're very clean designs that are individually kerned.
Try going to a local mall and you'll find demonstrations of this everywhere.
Here's a few popular examples:
The other most important thing you have to focus on is how the content of your text comes into play with what you surround it with.
This poster works well.
Take the word "Matrix".
When you use a word, get out a notepad and write down what the word makes you think of.
For the Matrix, the definition is "an environment or material in which something develops; a surrounding medium or structure". Additionally in mathematics, a matrix is "a rectangular array of quantities or expressions in rows and columns that is treated as a single entity and manipulated according to particular rules".
This poster works because it relates to what you'd think of in a matrix. The words are "glitchy" (you might relate that with numbers or computers, which word largely around maths). The characters and numbers in the background are in linear columns, similar to how a mathematical matrix is based in rows and columns. The font for the text is very "squared", and has very shark edges, like a square. The "glitchiness" in the font could also be thought of it being materialized, or coming together, similar to the first definition.
Placement of content is your next very important subject. Just because you have text doesn't mean it needs to be dead- center in the picture. Keep in mind where the eye is supposed to lead. Close your eyes, on a complicated typography picture. Open them. What do you see first? Where do your eyes go to next, and why?
For something like what you do, you want the text to be first thing you focus on. Don't make vignettes that make the edging letters hard to read; don't use fonts that are overly frilly. You want the viewer to be able to recognize what the word is, says, and how it interacts with the picture as quickly as possible. If a picture is hard to look at or is very convoluted, chances are the viewer will just skip right passed it and won't bother even looking at it. Complicated pictures are fine, but there's a fine line between complicated and overly complicated (and conversely, simplicity and oversimplifying).
A very good tutorial that talks about why text works in certain pictures and what to do to strengthen your text in your pieces can by found here by ctrlpaint (who's other tutorials are worth checking out; he's excellent with tutorials and explanation). This tutorial should wrap up about everything I talked about, I'd definitely recommend it.
Hope this helps, mate. You have a good start, and with more time and experience and studying, you'll do just fine.
EDIT: One more thing. Fonts, fonts, fonts.
Have a lot. You'll always find a use for different fonts.
Personally, I have a total of 2,334.
The only thing is, I wouldn't have much more than maybe 5,000. Even around how many I have, it really slows the font cache loadup time in Photoshop, and it usually takes a while to load up all of the text previews when clicking on the text tool in Photoshop.