You know how you're going to print your shirts. You have the equipment yourself or you know a printer that will give you a good rate. What you need is a design that will sell.
The first thing you should do is research, shop for tee shirts. The kinds of shirts you find for sale retail represent the kinds of shirts that sell well over all. Retail has a Embroidered workwear tight profit margin, there's little room for risk.
Now generally there are classic categories of designs that almost always sell well. The local pride design, for your state or local community. This sort of thing is a dependable seller. As are cute designs and masculine designs like those based on rock album covers. License artwork is also more of a sure thing, the commercial promotion of the cartoon or TV show will sell a tee shirt.
Next are the niche markets. The alternative band the Misfits has a very dependable selling design, being their logo. Beer drinking and marijuana smoking are popular counter culture themes and will sell shirts. Funny or sardonic ideas expressed on t-shirts will sell a t-shirt.
When it comes to niche markets, it's good to go with your gut. If you're a Christian, your Christian themed t-shirts may simply be better - it's something you know. It doesn't matter who you are, you are part of a subculture. If you love dogs, that's a subculture. If you're an Atheist or a Republican or Democrat - all of those represent niche markets that will sell t-shirts.
Now, it's quite likely you don't have the money to buy a license for commercial artwork. If you had the money to pay for a Teenage Mutant Ninja Turtle or Disney Snow White license, you would be less inclined to read this article.
So what do you do? Look for a vacuum, an idea that SHOULD exist, but doesn't - it's only a matter of time before someone else does it perhaps. That's what you want to design around.
There's a fella that's made millions off a stick figure and a funny platitude. Anyone could've done it, if they thought of the platitude and followed through.
Now, since you're making an investment you can't afford to take a great chance. But you can still innovate. Remember the popular themes for t-shirts are : civic pride, cute, masculine, commercial, and counter culture. If you can work one of these themes and create a fun twist to it, you can sell your shirts.
Yes, the idea is more important the the execution. The challenge of design is that you can only know it's good if YOU love it, but just because you love it doesn't mean anyone else will. So you should work up several ideas for tees, draw them out and test them on people. They'll tell you which ones are good, you'll know for sure when they offer to pre-order a design they want it so bad.
So, I'm a graphics professional. An idea that's strong enough will sell even a poorly designed t-shirt - but a well designed t-shirt with the same idea - will sell more.
So, let's say for sake of argument, you're an artist and have decided to create a Snow White design, but not the Disney version - the version you might find at an erotic dance club, a rated 'r' Snow White - maybe work in a street interpretation of 'snow' and imply a drug habit too . It takes a classic theme and gives it a counter culture twist. It could sell to hip boys and girls. After all, 'Snow White' is a public Embroidered workwear domain fairy tale, not the property of Disney.
That's as close as I can come to explaining commercial creative process to you. Take something that's already popular, and do something different with it. Such concepts could hit critical mass and make lots of money. One designer has gotten very wealthy from selling shirts with pictures of cute dogs on them, but drawn in a distorted style close to the tradition of Japanese Animation. Cute dogs are always popular, and this designer added a small twist to the theme is all.
Now, if you're not an artist, you could do the design by using a photograph of a model dressed as an erotic dancing Snow White. Just keep in mind public standards - of course. You want to sell a lot of shirts, not limit the scope to adult bookstores.