Nice bike! Looks very nice! I can't comment on the brand though as I'm in the US.

Gears: Generally you'll use the right gear lever to switch your rear "gears" (called cogs individually or together called a cassette) more often than the left lever. When you run out of rear gears, you switch the front gears (called chainrings.) The rear gears are spaced more closely together than the front chainrings. (So each gear "jump" is small on the back, but big on the front) You also want to avoid "cross chaining" as it will make it harder to pedal and you'll wear your gears and chain out quicker. (See this link HERE.) If your bike has 3 chainrings (3 gears up front) you'll probably mostly keep it in the middle chainring and just use the back to change gears. (That's generally how I ride my bike with three chainrings.) The bigger of the three only gets used when I'm going really fast or downhill, and the smaller only gets used when I'm going up huge hills or I'm loaded down with lots of stuff.) One last piece of advice for gears/chains. You have to lube your chain a LOT more than you'd imagine. I probably clean and lube mine once every two weeks or less depending on what I've ridden through lately.

Braking: Use both. Most braking force will be due to the front brake, but in slippery conditions you'll want to put a bit more emphasis on the back brake. Front tire skid = almost certain crash. Back tire skid = easily recoverable. Also, brake BEFORE turns. If you brake during turns traction is shared between braking and turning, and neither turns out well. I don't think this will matter too much on a city bike, but it's a good habit to get into.

Almost any bag designed to go on a rack will fit your rack. (There are certain bigger bags that will not fit made for for so called "extended bikes" or "xtracycles" but you do not have those bikes.) Or, if you want to go cheap, go find a box or milk crate and attach it to your rack with zip-ties or string/rope. You do have to be conscious of how big the bags are though. If they are too large your feet will hit them when you pedal. It's best to try the bag before you buy it if possible. (Or ask the bag company if it'll work well with your bike.) For small bags (called panniers) I wouldn't worry about your feet hitting them. Often times bags from reputable companies allow you to move the mounting points forward on the back therefore moving the bag further away from your feet.