This is a PDF eBook by Jeff Davies, which cover the very basics of photography, where you will learn what is depth of field, shutter speed, ISO and aperture, all that in a very simple and easy way.
Once you have read this ebook, you will not have anymore excuses to fail when you take a picture.
The thing to remember is that the smaller the f number, the shallower the depth of field.
Now, don’t run away with the idea that pictures taken at high ISO settings will be terrible, fit only for the bin or ‘delete’ key on your PC, because with modern cameras, ‘noise’ is acceptable, particularly if the file is printed to a relatively small size of say 15cm x 10cm or even bigger. But print the picture to 50cm x 40cm say, and the noise will be more noticeable.
The picture above shows a typical viewfinder and the information it displays.