How can I prevent chain rubbing my front derailleur, and how can I fix it ?

Brand new road bike I purchased three weeks ago and only put 50-60 miles on it, it only rubs when I'm in the higher resistant gears which allow me to go faster,once again it's the middle chain in the rear cause I'm riding a flat path, I need to fix it myself if possible because I cannot get to the bike shop for a couple weeks, any advice (shimano gear system, giant defy 3