Here's a thread from one of our FAQs. Read, comprehend, read some more!
In a nutshell, you crop to the proper ratio or specific dimensions in your crop tool as you are preparing for a specific print size. Don't save over the original, though, keep it and keep re-using it, in other words, "Save As" to get a dedicated copy for printing. It doesn't matter whether you crop using a ratio or specific dimensions, but exactly how you approach it will depend on the software on hand.
And, please don't cheat yourself by convincing yourself that aspect ratios are "too technical" for you! They are quite simple to grasp and once you do it becomes a piece of cake!
For your situation, the camera has a sensor with a 4:3 aspect ratio, I believe, meaning you can print a 3x4 inch print without cropping, or if you multiply those two figures you can print a 6x8, etc, without cropping. But for "standard" print sizes such as 4x6, 5x7 or 8x10 you will need to crop.
In your crop tool you should be able to enter those numbers and then grab the crop rectangle to the needed composition. Or, the 4x6 dimensions have a 2:3 aspect ratio. The 8x10 has a 4:5 aspect ratio -- note that you can just divide down the numbers into their simplest ratio. The 5x7 is easiest just leaving it at that for your crop dimensions.