All questions are good.
There are 9 cogs in the rear, and a triple crank up front, so you have 27 combinations, although using the smallest cog in the rear while on the largest crank in the front and vice versa (cross chaining), is not practical and can actually result in some unnecessary wear and tear to your cogs and chain.
Generally, the idea is to keep the angle of the chain in as straight a line is practical between the front cranks and rear cogs. As you become more familiar with your bike, you'll see that there may be some gear combinations you will seldom use, but if you find you need them, they'll come in handy!
Hope that helps