We often wonder as what Aperture value I should set, while taking a photo. Here are some really quick tips to help you there. These are of course not hard rule as photography is art and everything here boils down to your creative choice and imagination:
1. Lower F-number such as f/1.4, f/1.8, or f/2.8 produces shallow/narrow Depth of Field(DOF) and bigger F-numbers such as f/8, f/11, f/16 produces larger Depth of field.
2. To get candid portraiture shots, try to use minimum F-number you have got on your lens. This will give you a nice out of focus background (Bokeh).
3. If you are doing a group/family shot, then try to go for somewhere between f/5 to f/9 by evaluating the light available and number of people in the shot. As shallow depth of field may throw people standing backward rows out of focus.
4. If you are shooting a landscapes, then try for higher F-number such as f/9 to f/16 based on how much foreground, mid-ground and background you want in focus. Higher F-numbers would give you end to end sharpness (acceptable if not razor sharp). If possible try to stay away from highest F-numbers on your lens, as that might produce a softer image.
5. If you are doing a street photos in night and you are going after star burst effect, then try to shoot at any F-number more than F/11 and above.
6. And finally, every lens has a sweet spot, where it produces sharpest image. Its different for different lenses, but for most of the lenses its f/8 or f/9 ish. If you are not so keen on DOF and just want your images to be crisp while you are do running and gunning, you may stick to your lens's sweet spot. You may google to find the exact sweet spot for your lens and experiment from there to find really specific number just for your lens.
7. There are countless number of scenarios you may encounter, which I did not mention here. I leave that to your creativity and knowledge of photography techniques. However most of usual situation should be covered by above guidelines.
Hope it helps you find the F-number you shoot next time.