On the front-to-back: smallest chain ring (front) to largest cog (rear) will offer the easiest pedaling, but not much speed. As you go smaller rear, or larger front, pedaling gets harder, as speed goes up.

As far as shifting, if you have ridden more than 20-30 miles on a NEW bike, you may have gotten some cable stretch, which is normal for new cables, but will make shifting to the largest cog more 'iffy'. Your shop should do a free 'tuneup' as part of the purchase price--a good reason to buy from a LBS. If you bought it from someplace besides a Local Bike Shop, you are on your own for tuning it up. Not too hard for the rear--turn the cable adjuster at the Rear Derailleur about 1/4 turn counter-clockwise, and test ride/shift it. If not enough, try another 1/4 turn.