Shocks: the sag setting has to do with how much the shock gives with your weight. If it is an air shock, this is set with a special pump. There will be guidelines in the manufacturers manual on how many mm of sag you should have for your weight. The other setting is rebound. This is how fast/slow the shock responds or bounces back, and how it is set is both subjective and terrain dependent. For example, when I am riding on faster, smoother, flowy trails I have mine set pretty stiff, and on more technical stuff, slower rebounds. The front is also set up this way.

For handlebars, I would be very careful about changes, especially if you are new to Mtb. Wider will be more stable, but may feel strange at first. I have 22" bars on all my bikes.