I can only tell you what I know based on the T-shirt contest that I've been participating in here on dA.
Depending on the printshop, you might be limited to a certain number of colors because they would have to do more work to setup and print the T-shirt if it has a lot of colors or blends of colors. And as you know time is money.
I can't speak much of the resolution requirements seeing as none of the T-shirt contests that I've entered have so much as mentioned it. All I know is that there are various techniques used to make even small images look good on a T-shirt. However, since less quality is lost when resizing a larger image to something smaller, it's a good idea to make a decent size design, maybe something like 400px400px. But really someone else needs to answer this because I could be wrong.
If you happen have a vector program available, like Adobe Illustrator or (free) Inkscape, than resolution will not be an issue because vectors can be exported at any resolution without losing any quality.
BTW, since I'm currently in the midst of advertising my own T-shirt design, I won't post a link to it here but feel free to take a look at my signature. Anyway, I hope this info helps in some way.
I usually use Illustrator but Photoshop will do. Have your design at least at 150 dpi. I usually set it to 300 dpi. You don't really need InDesign though to create a t-shirt design.
Im guessing you've had stuff printed? Does your designs always come out right from the printers, like how you see it on the computer it comes out exactly like that on the shirt. I know alot of websites have a pop up screen where you just 'paste' your image on the virtual shirt but it seems to easy
I haven't gotten any shirts printed in person but I've sold about 10 t-shirts on RedBubble. I create logo designs as well. I usually use the same settings for logos and t-shirts after doing some research online. You have to be careful with colors. Some printers might not have the exact color that you have on the design so they use the color that they have that's close to the original. This usually works only with screen printers. Some places, like RedBubble, use Direct Printing methods so you can basically go all out with your colors and design. You just have to do a lot of reading and get well acquainted with graphic design and printing methods.
You might wanna produce your designs in illustrator, since its vector based and you won't need to worry about low-res nor pixelation. Once you have your designs ready, then save them as either an .eps or .ai file and import straight into InDesign to do the layout and finally print. Photoshop is limited when it comes to formatting your files and isn't ideal for print.
Nah brah. Its built for page layout, and exporting/pre-press files for print. I suppose if you gonna stick with photoshop you may wanna set your resolution to 300.
Basically are there any special requirements needed, like in photoshop do you need certain settings on, at a specific resolution, etc?