keep in mind that the finer (higher) the resolution (DPI) the better quality your image will be.
higher-end inkjet printers and most laser printers can change the dpi that they print at. in terms of printing, DPI is dots per inch. 100 dots per inch means that 100 dots of ink will fit in an inch of print space (per line). going down to 50 dots will make the ink dots bigger, and going up to 200 dots will make the dots smaller, allowing for more detail.
for printing regular text documents 72 dpi is usually good enough. for printing images, it depends on the image size and quality. if the image is huge, you can increase the dpi (which won't affect the size of the image on the computer) and the printer will print it differently.
300 is usually good enough, but you're printing on a t-shirt... how will it be printed? if it's printed straight onto the tshirt, there's the possibility that you can't even achieve that resolution.
Accept no substitutions.