Before I get into the process, you need to understand DPI in the printing industry. DPI stands for "Dots Per Inch". Standard web is at 72 DPI or pixels to be technical. Looks great on screen, but will look horrible on paper. Why you ask? Quite simple. The more dots in a tiny space, more detail is printed. That's why when making your prints, you should try to achieve at least 300 DPI.
|"pixels/inch" is your DPI, which needs to be 300.|
You can change the resolution very easily in Photoshop. Open your image and click on Image --> Image Size in the drop down list. At the bottom of the dialog box, uncheck Resample Image. Now, beside Resolution, type in 300. Re-check the Resample Image box and press ok. Just be warned that when you increase the resolution, your going to decrease the total image size, but more information is needed to print. I know it sounds a bit confusing, but it will work. Promise.