Welcome to the first baby-step in editing. If you want to follow along, instead of just reading this, feel free to grab a practice page from the "Cleaning" section of the "Practice/Test" tab.
First of all, if the scan you have is of two pages, you'll need to separate them. Just select half, Edit>Copy, File>New (OK), Edit>Paste. Next, you need to rotate the scan so that the majority of the black lines are straight. (I say majority because on many scans there will be one or more lines that don't line up with the rest. In this case, you need to eyeball the scan, and see which lines it's more important to set straight). The following is an incredibly easy way to rotate scans -- thanks Sephrenia & Dark'Heart for the tip!
Select the "Measure Tool", which is grouped under the "Eyedropper Tool" in photoshop.
Next, locate a vertical or horizontal line on your manga scan -- a line that should be straight. Click on one end of the line, and drag to the other end of the line and release. A gray line with plus symbols at its ends will have appeared following the line on the scan.
In the menu, go to Image>Rotate Canvas>Arbitrary.... In the box that appears, a value will automatically be entered. Hit okay. If you drew the Measure Line correctly, this should automatically rotate the scan straight.
As easy as this trick is, it's a good idea to double-check it. I suggest using the selection tool to draw a (straight) box around the line. If the lines are parallel, it's straight, and you're set. (Alternately, you can trigger a grid to appear on the screen with the shortcut Ctrl+", and line up the scan lines with that).
Continue to Cropping/Resizing ->