Even simpler. For a week or so don't shift the chain rings in front (left control) while riding. If you have 3 chain rings, get the chain onto the middle ring & leave it there. Shift the rear derailler so that the chain is on the middle or next biggest cog. That is where you will spend the most time. Go for a ride on flat ground and practice shifting the rear to the next smaller cog, then one smaller still and then back to the starting cog. Do this repeatedly until you are sure of what will happen when you move your shifters, then try shifting 1 or 2 cogs larger and back. One of the mysteries of bike gearing is that you may have "27 gearings" but you don't have 27 different gears. There is a lot of overlap. If you are just starting to ride, you won't need to use your front derailleur much, the middle ring will do you fine. Once you are really comfortable with shifting the rear gears and are confident on the flat, start some hill work. Only after you are comfortable climbing and descending in the middle ring do you want to start shifting to the big ring on the flats and granny ring on the hills.