These three simple steps that take less then 30 seconds to complete, will take any of your old photos off your camera or phone and improve them significantly with minimal other editing. The last step is in my opinion the one that gets forgotten about the most but can improve the way your image is portrayed significantly.
1. Exposure - Use the exposure slider to either brighten or darken the exposure of the photo to where you want it to be.
Typically if shooting in any Auto or semi-Auto mode, the camera takes it's best guess as to how the final image should look. Since there's no real right or wrong answer, adding your own input will set the image exposure exactly right where You feel it should be.
2. Contrast - Once you've either lightened or darkened the photo to where you feel the image should be, adjust the contrast to suit. Sometimes this slider can be left alone entirely.
3. Cropping & Straightening - It sounds so simple and it really is. Making sure that the photo is straightened and then cropped perfectly will make it look like you really put a lot of thought and effort (which you likely did) into capturing the perfect shot.
Below are four quick examples of photos that look like they might have been missed opportunities but that can be quickly brought up to decent shape with just the there basic tweaks mentioned above. It doesn't hurt that they're all shot in RAW format either. ;)