Tom
Glad you are making progress.
I am very lucky in that I spend most of my time in Africa where the light is usually pretty good. But despite that I operate almost exclusively on ISO400. With that you can usually operate on a reasonably fast shutter speed (never below 1/500 and ideally above 1/1000). Aperture although secondary to shutter speed is also important. My lenses (all excellent Minolta G ones) seem to be crispest at F8 or higher (smaller aperture = using middle flatter part of the lens so depth of field is less critical - ie both nose and tail might be in focus at the same time!).
With the modern digitals just experiment with different settings but above all get used to your camera. I have taken some 30,000 pics with my digital bodies and know it inside out etc. Vital as things sometimes happen fast.
Re a speck of dust on the sensor you will need to consult the manual re cleaning but advice learnt the hard way is only change lenses in a reasonably 'dust free' environment. In other words not outside at an airfield in high winds and dust everywhere!
T24