How do I effectively use my 7-speed Schwinn mountain bike?

Training for the Danskin triathalon and I am having trouble adjusting my gears to the right ones for up hill, downhill and straightaway. It's been ages since I have rode a bike and don't think I had gears then. What is the right side vs. left side and what should it be on to go uphill, straight away and downhill?

Right Side is for the front, between your feet.
Left side is for the rear smaller cogs.
Up hill should be small at front and big at rear.
Down hill should be big at front and small at rear.
Straight away should be in between and pedal comfortable but quite quickly, most people use a gear a little too hard. Take a look at a Lance Armstrong video to get an idear of pedalling quickly.

For training practice riding at the pace you want to race at, but for short periods and lots of rests. As you get better you can extend the periods and reduce the rests. Have a day off every 3rd day.

You can use the bike in a lot of situations, it's built for versatility.

First off, most of your riding will be done in the middle chainring-the gears on the pedals. That's like "D" in your car. The smaller one is for climbing, but you're going to want to stay in the lower 3-4 gears (the bigger cogs on the cassette) with the smaller ring. More to come on that. The larger ring is for speed, but you're going to want to stay in the higher 3-4 gears.

The reason you want to limit your shifting on the smaller and larger chain ring is because of a physics issue with bikes called cross chaining. If you look down on your bike with the chain in the small chain ring and high gears or reverse, you will see that the chain is diagonal on the gears. That equals tension on the chain which is bad for two reasons. First, the chain will scrape on the front derailleur. Second it will put tension the chain to the point of stretching it or in the extreme breaking it. My wife did that last weekend.

Also see the article below on gearing from Sheldon Brown. He's the Might Oz of bike information! I've also included a gear calculator-gear ratios

Bottom line is you have a versatile bike, you'll probably ride 60% or more in the middle chain ring and ride comfortably.

Good luck and happy riding!

uphill - smallest gear in front largest gear in rear
straight/downhill - largest gear in front and smallest o=in the rear.