The best way to ride better is to ride more--it's that simple. It may take a few weeks of regular (four or five times a week) rides to notice the difference, but it WILL come. Try not to kill yourself at first--do what you're comfortable with, take a day off (or do a really easy ride) when you need to, and add maybe 10 percent a week, no more. Don't push so hard you burn out or start to hate it. I remember my first 10-mile ride, more than 30 years ago, and thinking I was a hardened cyclist. Now I ride 25 to work without giving it a thought.
A few beginner tips, though:
--A Target bike is probably very low end, which is OK for where you are now, but it wouldn't hurt to check it over to make sure everything is tight, the tires are inflated properly etc. I used to build bikes for a department store when I was in high school, and it's a wonder we didn't kill a dozen people every Christmas. Lubricate everything that moves, too--the chain, the pivot points in the brakes and derailleurs etc. It probably wasn't done right at the factory and it certainly wasn't done in the store. A good Web site for general information is www.parktool.com.
--Get a patch kit, tire levers and a pump, learn to use them and carry them with you. You WILL have flat tires; I've had three this week. If you need a wrench to get your wheels off (if they're not quick-release), carry that, too. You can be back on the road in 10 minutes if you have the right stuff.
--Big fat cushy seats are often less comfortable than hard ones on longer rides, but if that's what you've got, make the most of it. Be sure it's set level with the ground, or nearly so, and adjust it in tiny amounts (fractions of an inch of tilt) until it feels right. Moving the nose of the saddle an eighth of an inch up or down can make a difference.
--Learn to use the gears properly. In general, no matter what you're doing (uphill, flat or down) you want to be in a gear that lets you pedal at 80-100rpm (complete revolutions of the cranks, 360 degrees) all the time. Going uphill, use the small chainring and a large rear gear. On the flat or downhill, shift to a larger ring in front and/or a smaller one in back. As another post said, find a flat, quiet spot and practice. you can learn this in about 10 minutes.
And for God's sake, have fun. This is just a form of recreation--don't make it a chore or a challenge.