I don't know what those numbers mean. For flat roads and no wind, start in the middle ring in the front and somewhere in the middle of the cog in the rear. Don't think of it as 21 speeds; think of it as 3 ranges of 7 speeds. The big outside ring is high range for when you are going downhill or have a tailwind. The middle ring is normal range for most flattish riding. The small inside ring is your granny gear.

Keep at it, and if you feel like you're at your limit, then back off a bit.