Heed Joey P. Monitor calibration is essential as it’s the only way to be sure you’ll get reliable colours. Most pros and dedicated amateurs use an external device that attaches to the front of your screen and measures the actual light wavelengths in order to properly adjust the monitor. I use this one. If you don’t want to spend the money, then you can try to calibrate with Adobe Gamma or the MS Colour control applet.
As for photoshop post-processing, the most critical aspects are resolution, contrast and sharpening. Since you mention cropping, make sure that you have at least 250 ppi (300 ppi is even better), at the size you wish to print.
Secondly, if you don’t know how to read a histogram, then learn! Check out Understanding Histograms from Luminous Landscape (an excellent photography resource web site). Once you’ve got a handle on this, not hard, then use levels to adjust the contrast in your photo. Ideally, you’ll want a nice range of tones from black to white, although there are exceptions. Be careful not to overdo it and blow out your highlights or turn your shadows completely black.
As for sharpening, well, sharpening is a little bit of a black art. Generally, all digital photos require sharpening. Most digital cams apply at least some of it in-camera to your JPGs, but you’ll still want to some output sharpening for the printer. Here’s a good Sharpening 101 article by Thom Hogan. Unfortunately, it’s a complex subject and requires much trial and error before you can really get a handle on it. One thing to keep in mind is that you want your photo to look just a little TOO sharp on-screen, before sending to the printer.
It’s definitely worth it to get just a few prints made first so you can adjust your files accordingly. Also, have a look around to see if there are any local camera stores that have printing services. It’s almost guaranteed that the prints they produce will be better than Walmart’s and they probably won’t be that much more expensive either.
Oh, BTW, NEVER work on your original file. Always work on a backup.